PRINCIPLES  OF 
INTERCHANGEABLE  MANUFACTURING 


PRINCIPLES   OF 

INTERCHANGEABLE 

MANUFACTURING 


A  TREATISE  ON  THE  BASIC  PRINCIPLES 
INVOLVED  IN  SUCCESSFUL  INTERCHANGE- 
ABLE MANUFACTURING  PRACTICE 
COVERING  DESIGN,  TOLERANCES,  DRAW- 
INGS, MANUFACTURING  EQUIPMENT, 
GAGING  AND  INSPECTION 


BY 
EARLE    BUCKINGHAM,    A.S.M.E.,    S.A.E. 

ENGINEER,  PRATT  &  WHITNEY  Co. 


FIRST  EDITION 
FIRST  PRINTING 


NEW  YORK 
THE   INDUSTRIAL   PRESS 

LONDON:  THE  MACHINERY  PUBLISHING  CO..  LTD 


COPYRIGHT,  1921, 

BY 

THE  INDUSTRIAL  PRESS 
NEW  YORK 


COMPOSITION  AND  ELECTROTYPING  BY  THE  PLIMPTON  PRESS,  NORWOOD,  MASS.,  U.S.A. 


PREFACE 

WHILE  many  articles  dealing  with  various  phases  of  inter- 
changeable manufacturing  have  appeared  from  time  to  time 
in  the  technical  press,  no  complete  and  comprehensive  treatise 
dealing  with  this  subject  as  a  whole  has  heretofore  been  avail- 
able to  those  interested  in  interchangeable  manufacturing  in 
the  machine  building  and  metal  working  fields. 

The  development  of  interchangeable  manufacturing  is  closely 
interwoven  with  many  distinctly  American  manufacturing 
methods  and  processes.  Every  large  American  industry  has 
contributed  its  share  to  the  progress  made  in  interchangeable 
manufacturing.  Different  plants  working  along  independent 
lines  have  often  achieved  the  same  results  by  widely  different 
methods.  The  author  has  attempted  to  define  and  emphasize 
the  underlying  basic  principles,  using  specific  methods  only 
when  necessary  to  illustrate  the  application  of  these  principles 
in  actual  manufacturing  processes.  He  has  gathered  the  in- 
formation upon  which  this  treatise  is  based  from  many  manu- 
facturing plants,  both  large  and  small,  in  this  country  and  in 
Canada.  He  has  seen  every  method  discussed  in  successful 
operation,  some  in  one  plant,  some  in  another  —  but  not  all 
in  any  one. 

For  more  than  ten  years  the  author  has  been  in  constant 
touch  with  many  of  the  detailed  manufacturing  problems  that 
arise  in  the  production  of  interchangeable  mechanisms  in  large 
quantities.  During  the  World  War  his  work  took  him  for  four 
years  into  many  manufacturing  plants  in  connection  with  ord- 
nance work,  first  for  private  corporations  and  later  for  the 
Ordnance  Department.  When  engaged  in  this  work  it  became 
apparent  to  him  that  the  absence  of  common  methods  of  inter- 
pretation of  drawings,  tolerances,  and  specifications,  the  lack 
of  uniform  gaging  methods  and  misunderstanding  of  many  of 

v 

822419 


VI  PREFACE 

the   factors   of   interchangeable   manufacturing,   presented   an 
urgent  need  for  a  complete  treatise  on  this  subject. 

In  arranging  the  material  available  on  the  subject  of  inter- 
changeable manufacturing,  the  author  has  first  taken  up  the 
general  principles  involved  in  the  industrial  application  of  this 
method  of  production,  and  has  then  devoted  a  separate  chapter 
to  the  definition  of  the  terms  used,  so  that  there  will  be  no  mis- 
understanding as  to  the  meaning  of  the  terms  used  later  in  the 
book.  The  influence  of  interchangeable  manufacturing  processes 
on  machine  design  and  the  purposes  of  models  are  then  dealt 
with,  followed  by  a  complete  and  minute  discussion  on  the 
dimensioning  of  drawings  intended  for  use  in  interchangeable 
manufacturing.  This  is  followed  by  a  discussion  of  the  principal 
elements  that  govern  mechanical  production,  the  equipment 
required  for  interchangeable  manufacturing  (including  machines, 
jigs  and  fixtures);  the  gaging  equipment  necessary;  and  the 
principles  of  inspection  and  testing.  Special  chapters  are  also 
devoted  to  the  manufacture  for  selective  assembly,  and  methods 
used  in  small  quantity  production  on  an  interchangeable  basis. 
An  entire  chapter  deals  with  the  service  factor  in  interchangeable 
manufacturing,  because  in  the  final  analysis  no  manufactured 
machine  or  device  is  ever  purchased  for  itself  alone,  but  is 
acquired  for  the  service  which  it  is  supposed  to  render. 

The  Pratt  &  Whitney  Co.,  Hartford,  Conn.,  with  whose 
cooperation  this  treatise  is  written,  submits  it  to  the  public 
as  a  part  of  the  company's  contribution  to  the  art  of  inter- 
changeable manufacturing  with  the  hope  that  it  will  assist 
manufacturers  and  mechanics  to  employ  effectively  the  prin- 
ciples of  interchangeable  manufacturing  and  to  reap  the  benefits 
that  a  rational  application  of  these  principles  make  possible. 
The  author  also  wishes  to  acknowledge  at  this  time  the  assist- 
ance that  has  been  given  him  by  many  other  manufacturing 
plants  that  he  has  visited.  To  name  them  all  would  mean  a  long 
list  of  prominent  plants  manufacturing  machine  tools,  auto- 
mobiles, tractors,  ordnance,  typewriters,  watches,  phonographs, 
instruments,  etc. 

EARLE  BUCKINGHAM 


CONTENTS 


CHAPTER  I 

PRINCIPLES  OF  INTERCHANGEABLE 
MANUFACTURING 

PAGES 

Economy  —  Extent  of  Interchangeability  —  Clearances  — 
Tolerances  —  Component  Drawings  —  Specifications  —  Gages 
for  Checking  Results  —  Manufacturing  Equipment  —  Pro- 
duction Problems  —  Inspection  of  Product 1-17 

CHAPTER  II 

TERMS  USED  IN  INTERCHANGEABLE 
MANUFACTURING 

Interchangeability  -  -  Selective  Assembly  -  -  Function  — 
Limit  —  Tolerance  —  Basic  and  Model  Size  —  Maximum  and 
Minimum  Metal  Size  —  Maximum  and  Minimum  Clearance  — 
Interference  —  Operating,  Functional,  and  Clearance  Sur- 
faces —  Elementary  and  Composite  Surfaces  —  Compound 
Tolerances  —  Register  Points  —  Unit  Assembly  —  Component 
and  Operation  Drawings 18-28 

CHAPTER  III 

MACHINE  DESIGN  IN  INTERCHANGEABLE 
MANUFACTURING 

Classes  of  Design  —  Simplifying  Design  —  Choice  of  Ma- 
terials —  Clearances  and  Tolerances  —  Application  of  Inter- 
changeable    Principle  —  Advantages     of     Unit    Assembly  - 
Designing  for  Assembling  and  Service 29-39 

CHAPTER  IV 
PURPOSE  OF  MODELS 

Manufacturing  Model  to  Test  Functioning  —  Experimental 
Model  —  Testing  Tolerances  —  Model  for  Standard  of  Pre- 
cision    40-45 

vii 


viii  CONTENTS 

CHAPTER  V 

PRINCIPLES  IN  MAKING  COMPONENT  DRAWINGS 

PAGES 

Functional  Drawings  —  Manufacturing  Drawings  —  Laws  of 
Dimensioning  —  Inspection  Gage  Requirements  —  Composite 
Surfaces  —  Compound  Tolerances  —  Force  Fits  —  Profile  Sur- 
faces —  Dimensioning  Holes  —  Location  of  Holes  —  Con- 
centricity and  Alignment  —  Gears 46-76 

CHAPTER  VI 
PRACTICE  IN  MAKING  COMPONENT  DRAWINGS 

Maintaining  Functional  Requirements  of  a  Mechanism  — 
Basic  Dimensioning  —  Maintaining  a  Common  Locating  Point 
—  Length  Dimensions  from  Common  Locating  Point  —  Draw- 
ing of  Separate  Parts  —  Compound  Tolerances 77-104 

CHAPTER  VII 
ECONOMICAL  PRODUCTION 

Specifications  —  Functions  and  Requirements  of  Product  — 
Manufacturing  Data  —  Factory  Cost  —  Direct  Labor  Cost  — 
Machine  Hour  Rate  —  Product  Overhead  —  Clerical  and  Ac- 
counting Work  —  Specific  and  General  Information 105-120 

CHAPTER  VIII 

EQUIPMENT  FOR  INTERCHANGEABLE 
MANUFACTURING 

Selection  of  Machine  Tools  —  Designing  Jigs  and  Fixtures  — 
Cutting  Tools  —  Locating  Points  —  Chip  Clearances  —  Check- 
ing and  Testing  Jigs  and  Fixtures  —  Maintaining  Tolerances  — 
Special  Equipment  for  Machining  Automobile  Transmission 
Cases  —  Drilling  Holes  Simultaneously  —  Pneumatic  Clamp- 
ing Devices  —  Multiple-Tool  Facing  Bar  —  Milling  and  Drill- 
ing Fixtures 121-176 

CHAPTER  IX 
GAGES  IN  INTERCHANGEABLE  MANUFACTURING 

Classification  According  to  Use  —  Accuracy  —  Working  and 
Inspection  Gages  —  Interchangeability  between  Parts  made  in 


CONTENTS  IX 

PAGES 

Different  Shops  —  Snap,  Ring,  and  Plug  Gages  —  Contour  or 
Profile  Gages  —  Receiving  Gages  —  Flush-pin  Gages  —  Sliding 
Bar    Gages  —  Depth    and    Length    Gages  —  Hole    Gages  - 
Gaging  Threads  —  Tolerances  on  Threaded  Parts  —  Wing  and 
Indicator    Gages  —  Functional     Gages  —  Gaging    Gears  - 
Master  and  Reference  Gages 177-216 

CHAPTER  X 
INSPECTION   AND  TESTING 

Discrepancy  between  Part  and  Drawing  —  Incomplete 
Drawings  and  Specifications  —  Shop  Inspection  —  Final  In- 
spection —  Inspecting  Gages  and  Material  —  Testing  As- 
sembled Mechanisms 217-223 

CHAPTER  XI 
MANUFACTURING  FOR  SELECTIVE  ASSEMBLY 

Clearances  and  Tolerances  —  Dimensions  and  Tolerances  on 
Drawings  —  Laws  of  Dimensioning  — Similarity  of  Specifica- 
tions, Equipment,  Gages,  and  Inspection  Methods 224-225 

CHAPTER  XII 
SMALL-QUANTITY  PRODUCTION  METHODS 

Standardization  of  Nominal  Sizes  —  Clearances  and  Toler- 
ances —  Economy   of    Standardization  —  Standardizing   Unit 
Assemblies  to  Suit  Several  Machines  —  Component  Drawings 
-  Manufacturing  Equipment  —  Gages  and  Methods  of  In- 
spection   230-240 

CHAPTER  XIII 

SERVICE  FACTOR  IN  INTERCHANGEABLE 
MANUFACTURING 

Functional  and  Manufacturing  Designs  —  Keeping  Specifica- 
tions up  to  Date  —  Planning  Production  to  Obtain  Required 
Service.  .  241-245 


PRINCIPLES 

OF   INTERCHANGEABLE 
MANUFACTURING 


CHAPTER  I 
PRINCIPLES   OF   INTERCHANGEABLE   MANUFACTURING 

INTERCHANGEABLE  manufacturing  consists  of  machining  the 
component  parts  of  a  given  mechanism  in  the  manufacturing 
departments  within  such  limits  that  they  may  be  assembled  in 
the  assembling  department  without  fitting  or  further  machining. 
>  Component  parts  may  also  be  replaced  or  transferred  from 
one  mechanism  to  another  without  detriment  to  the  function- 
ing and  without  machining.  The  advantages  of  such  a  method 
of  manufacture  are  self-evident,  and  need  not  be  dwelt  upon 
further.  It  is  obvious  that  with  proper  equipment  and  control, 
the  component  parts  of  a  mechanism  can  thus  be  manufactured 
in  large  quantities  at  a  low  direct  labor  cost. 

Economy  of  Interchangeable  Manufacturing.  In  all  private 
industrial  enterprises  ultimate  economy  is  the  controlling  factor 
of  any  method  of  procedure.  This  does  not  necessarily  mean 
that  the  methods  adopted  always  are  actually  the  most  economi- 
cal. Methods  which  will  promote  this  economy  are,  however, 
the  ideals  toward  which  manufacturers  are  constantly  striving. 
Now,  a  careful  analysis  will  show  that  interchangeability  does 
not  always  result  in  ultimate  economy.  In  such  cases  the  at- 
tempt to  maintain  it  is  a  fault,  not  a  virtue. 

To  make  this  point  clear,  consider  the  matter  first  from  the 
standpoint  of  production  alone.  The  equipment  and  prepa- 
ration necessary  to  produce  interchangeable  parts  are  expensive. 
In  making  only  a  small  number  of  special  mechanisms,  it  would 
be  gross  extravagance  to  maintain  any  high  degree  of  inter- 
changeability.  Viewed  simply  as  a  question  of  production,  the 


2         ;  .;  :\  .INTERCHANGEABLE   MANUFACTURING 

problem  of  interchangeable  parts  is  solved  by  establishing  a 
balance  between  manufacturing  and  assembling  costs,  whether 
the  quantity  of  production  be  great  or  small,  whether  the  mecha- 
nism involved  be  a  standard  or  a  special  product. 

Ultimate  economy,  however,  must  include  the  factor  of  service. 
Suppose  automobiles,  typewriters,  sewing  machines,  or  sporting 
rifles  are  sold.  Parts  will  wear  out  or  be  broken  by  accident. 
The  maintenance  of  service  stations,  where  extra  parts  are 
quickly  available,  tends  to  keep  customers  satisfied.  Service 
stations  will  be  least  expensive  if  the  product  is  truly  inter- 
changeable and  the  agent  can  replace  a  part  with  the  aid  of  a 
screwdriver  or  wrench  —  or,  still  better,  if  the  customer  can 
replace  it  himself.  Since  the  advent  of  the  automobile,  people 
have  been  much  more  interested  in  things  mechanical  than 
before,  and  have  taken  pride  in  making  their  own  repairs.  The 
more  nearly  interchangeable  mechanisms  are  made,  the  more 
this  desirable  trait  is  fostered  and  the  less  will  service  stations 
cost.  Ultimate  economy,  then,  requires  that  service  costs  be 
balanced  against  total  productive  costs. 

Degree  of  Interchangeability  Desirable.  It  should  not  be  as- 
sumed from  this  that  entire  interchangeability  or  none  at  all 
must  be  had.  In  almost  every  mechanism  certain  parts  are 
begun  as  separate  units  in  order  to  simplify  the  manufacture, 
but  are  later  permanently  assembled  into  a  single  unit  and 
machined  to  completion  as  such.  In  many  such  cases,  the 
expense  of  attaining  interchangeability  would  be  too  great  to 
justify  the  attempt,  because  of  the  many  mechanical  difficulties 
to  be  overcome.  It  would  be  more  economical,  in  case  of  break- 
age, to  discard  and  replace  the  entire  assembled  unit.  In  other 
cases,  the  functional  requirements  may  be  so  severe  that  a 
system  of  selective  assembly  will  prove  to  be  the  proper  course, 
although  this  entails  carrying  a  double  or  triple  number  of  spare 
parts  in  service  stations,  or  involves  some  fitting  when  replacing 
unserviceable  parts. 

In  general,  however,  interchangeability  is  a  desirable  goal, 
and  is  readily  attained  in  the  majority  of  cases  if  the  proper 
attention  is  given  to  the  basic  principles  governing  it,  including 


INTERCHANGEABLE   MANUFACTURING  3 

the  design  of  the  mechanism  and  the  process  of  manufacture; 
yet  it  is  limited  in  several  directions  by  the  inadequacy  of  many 
present  manufacturing  conditions.  With  improved  facilities, 
it  may  be  that  in  future  years  a  much  greater  degree  of  inter- 
changeability  will  be  possible  than  at  present. 

The  following  paragraphs,  which  are  based  on  manufacturing 
conditions  as  they  now  exist,  trace  the  progress  of  a  commodity 
through  all  stages  of  its  manufacture,  from  its  inception  as  a 
mechanical  project  to  the  final  testing  that  determines  its  suc- 
cessful completion.  An  attempt  has  been  made  to  single  out  for 
special  comment  those  factors  which  make  possible,  or  promote, 
the  interchangeable  manufacture  of  its  parts. 

Design  as  it  Affects  Success.  The  development  of  any  new 
mechanism  starts  with  a  mental  conception  of  some  function 
to  be  performed.  This  conception  then  takes  detailed  form, 
first  mentally,  then  on  paper,  and  finally  in  metal.  The  experi- 
mental model  —  if  such  be  constructed  —  is  usually  made  by  the 
cut-and-try  method.  Little  attention  is  paid  in  the  beginning 
to  future  manufacturing  requirements.  The  main  object  is  to 
construct  a  mechanism  that  will  function  properly  regardless 
of  the  exact  design.  When  this  end  is  reached,  what  may  be 
called  the  inventive  or  functional  design  has  demonstrated  its 
success. 

Before  manufacturing  is  begun,  however,  a  manufacturing 
design  must  be  perfected  which  will  modify  the  inventive 
design  so  as  to  allow  its  economical  production  on  a  large  scale. 
Several  manufacturers  recognize  this  twofold  nature  of  design- 
ing, and  maintain  a  separate  department  for  each  type.  In- 
dispensable as  is  the  original  invention,  it  is  the  manufactur- 
ing design  which  largely  determines  the  success  or  failure  of  a 
given  project.  This  manufacturing  designing  necessarily  con- 
tinues throughout  the  whole  course  of  production  because  of  the 
almost  infinite  number  of  petty  detailed  questions  involved, 
only  a  few  of  which  can  be  foreseen  and  answered  in  advance. 
One  of  the  important  functions  of  an  engineering  department  is 
to  keep  in  close  touch  with  the  progress  of  the  work  in  the 
shops,  deduce  general  principles  therefrom,  and  apply  these 


4  INTERCHANGEABLE   MANUFACTURING 

principles  not  only  to  the  work  in  hand,  but  also  to  all  new  work 
that  may  be  developed. 

The  Manufacturing  Model.  Assume  that  the  functional  re- 
quirements of  the  mechanism  are  established  and  that  the 
manufacturing  design  has  been  adopted.  The  first  concern  is 
to  test  this  design  as  far  as  possible.  The  most  certain  method 
of  accomplishing  this  is  to  develop  a  physical  model.  Such  a 
model  must  not  be  confused  with  the  experimental  model, 
as  its  purpose  is  quite  different.  The  experimental  model 
shows  that  the  mechanism  will  perform  certain  functions. 
The  manufacturing  or  physical  model,  if  properly  developed, 
proves  that  the  mechanism,  as  modified  and  developed  to  facili- 
tate manufacture,  still  retains  the  functional  advantages  of  the 
experimental  model.,,  The  manufacturing  model  is  naturally 
an  expensive  piecei  of  equipment,  but  if  a  large  output  of  a  new 
commodity  is  under  consideration,  it  is  money  well  invested. 
In  the  case  of  a  small  total  output,  a  "pilot"  mechanism  is  often 
built  for  this  purpose,  which  is  not  set  aside  for  future  reference 
but  incorporated  in  the  product  itself. 

There  are  many  other  services  which  a  manufacturing  model 
is  capable  of  rendering.  It  may  serve  as  a  physical  standard  of 
dimensions  for  the  future  product.  In  this  case,  it  must  be  made 
with  much  greater  care  than  if  it  were  to  be  used  merely  to  test 
the  functioning  of  the  manufacturing  design.  Such  a  model 
will  be  of  great  value  as  a  reference  at  all  times  during  pro- 
duction: In  itself,  it  comprises  an  effective  functional  gage  to 
test  any  completed  part.  It  should  be  used  but  rarely,  however, 
for  that  purpose.  In  addition,  the  component  parts  of  the  model 
are  of  great  assistance  in  checking  the  manufacturing  equipment 
in  the  early  stages  of  the  work. 

Clearances.  Clearances  are  vital  factors  in  interchangeable 
manufacturing.  Fits  can  be  secured  without  interchangeability, 
but  the  latter  cannot  be  maintained  without  proper  clearances. 
It  is  self-evident  that  a  certain  space  must  be  left  between 
operating  parts.  The  minimum  clearances  should  be  as  small 
as  the  assembling  of  the  parts  and  their  proper  operation  under 
service  conditions  will  allow.  The  maximum  clearances  should 


INTERCHANGEABLE   MANUFACTURING  5 

be  as  great  as  the  functioning  of  the  mechanism  permits.  The 
variation  between  a  maximum  and  a  minimum  clearance  de- 
termines the  manufacturing  tolerance.  It  is  clear,  then,  that 
determining  at  the  outset  the  permissible  clearances  establishes 
also  the  extent  of  the  tolerances  which  control  the  final  inspection. 

Clearances  should  be  one  of  the  principal  considerations  in 
developing  the  manufacturing  design.  This  design  should  aim 
to  allow  the  greatest  possible  amount  of  clearance  between 
companion  parts.  The  more  the  design  lends  itself  to  this  end, 
the  greater  the  economy  of  manufacture  and  the  greater  the 
degree  of  interchangeability  obtainable.  In  determining  which 
parts  of  a  mechanism  can  be  made  interchangeable,  this  matter 
of  permissible  clearances  plays  the  largest  part.  A  mechanism 
which  is  so  designed  that  it  cannot  permit  fairly  liberal  clear- 
ances is  not  a  suitable  one  to  be  manufactured  on  a  strictly 
interchangeable  basis  with  the  standard  equipment  now  avail- 
able. Every  operating  part  of  a  mechanism  must  be  located  with- 
in reasonably  close  clearances  in  each  plane.  After  such  require- 
ments of  location  are  met,  all  other  surfaces  should  have  liberal 
clearances,  unless  the  factor  of  strength  is  the  controlling  one. 

Manufacturing  Tolerances.  The  general  tendency  in  the  past 
has  been  to  establish  manufacturing  tolerances  by  trying  to 
hold  the  product  as  closely  as  possible  to  a  fixed  size.  The  natural 
result  of  this  policy  is  that  the  tolerances  established  on  paper 
are  often  exceeded;  yet  the  actual  working  variations  remain 
unrecorded,  because  it  is  argued  that  under  certain  conditions 
the  original  requirements  might  be  met  and,  therefore,  the 
tolerances  noted  are  the  proper  ones,  even  though  they  are 
not  maintained.  Every  effort  to  make  the  recorded  tolerances 
represent  the  actual  working  tolerances  is  opposed  on  the  ground 
that  such  a  procedure  would  lower  the  shop  standards.  As  a 
matter  of  fact,  it  is  hard  to  understand  how  anything  could 
lower  the  standards  of  the  shop  more  than  the  absolute  disregard 
of  the  rules  it  is  supposed  to  be  obeying. 

There  is  a  further  argument  for  the  acceptance  of  liberal 
tolerances.  Too  often  in  manufacturing  concerns,  and  especially 
in  the  case  of  interchangeable  manufacturing,  one  finds  details 


6  INTERCHANGEABLE   MANUFACTURING 

being  made  ends  in  themselves  rather  than  means  to  a  larger 
end.  In  producing  a  component  part,  the  main  object  should 
not  be  to  demonstrate  how  closely  a  fixed  size  can  be  approached; 
the  aim  should  be  to  construct,  as  economically  as  possible,  a 
mechanism  that  will  satisfactorily  perform  certain  functions. 
The  knowledge  of  how  accurately  a  machining  operation  can 
be  performed  is  indeed  invaluable  in  making  the  manufacturing 
design;  but  when  that  design  has  once  been  completed,  interest 
should  shift  to  the  proper  functioning  of  the  completed  mecha- 
nism. Finally,  it  may  be  said  that  in  most  cases  the  tolerances 
originally  fixed  are  increased  during  the  process  of  manufactur- 
ing without  detriment  to  the  mechanism.  It  is  rarely  that  a 
tolerance  has  to  be  reduced. 

The  proper  minimum  clearances  can  be  determined  quite 
readily  and  definitely  for  most  cases  in  the  early  stages  of  the 
work  —  the  manufacturing  model  is  of  great  value  in  this 
respect  —  but  the  maximum  clearances  become  established  only 
after  extended  experience  with  the  particular  mechanism.  In 
many  cases  the  extreme  maximum  is  never  found,  because 
long  before  that  point  is  reached,  the  tolerances  have  become  so 
liberal  that  there  is  no  need,  from  the  standpoint  of  economical 
production,  to  increase  them  further. 

Component  Drawings.  Component  drawings  have  two  main 
functions  to  perform.  The  first  is  to  give  such  information  about 
the  design  and  the  tolerances  that  the  manufacture  of  the  product 
can  begin.  This  does  not  seem  like  a  very  difficult  task,  but  the 
notation  of  the  tolerances  on  component  drawings  has  created 
new  problems  of  interpretation  that  have  not,  as  yet,  been 
fully  solved.  At  the  present  time,  the  language  of  drawings  is 
not  altogether  clear  and  exact. 

The  first  tendency  in  introducing  tolerances  on  drawings 
seems  to  have  been  to  attempt  to  express  a  permissible  variation 
on  every  dimension  given.  The  results  obtained  in  the  shop 
depend,  then,  upon  the  particular  combination  of  dimensions 
used.  Different  organizations  using  different  combinations 
could  obtain  radically  different  results;  and  of  the  possible 
number  of  different  combinations  there  is  no  end. 


INTERCHANGEABLE    MANUFACTURING  7 

The  existence  of  a  tolerance  on  a  drawing  is  an  acknowledg- 
ment that  variations  are  inevitable  in  the  physical  dimensions 
of  the  product.  Any  dimension  given  on  such  a  drawing  without 
a  tolerance  should  not  be  construed  to  denote  an  absolute  size 
without  error,  but  rather  to  indicate  either  that  the  permissible 
variation  for  that  point  or  surface  is  controlled  by  tolerances 
given  on  other  co-related  dimensions,  or  that  the  dimension  is 
so  relatively  unimportant  that  no  attempt  had  been  made  to 
determine  its  permissible  variation. 

In  making  component  drawings,  the  effort  should  be  made 
to  so  give  the  dimensions  and  necessary  tolerances  that  it  would 
be  possible  to  lay  out  one,  and  only  one,  representation  of  the 
" maximum  metal"  condition  and  one,  and  only  one,  of  the 
" minimum  metal"  condition.  If  such  lay-outs  were  super- 
imposed, the  difference  between  them  would  represent  the 
permissible  variation  on  every  surface.  Any  condition  of  the 
product  which  fell  within  the  zone  thus  established  could  be 
considered  as  meeting  the  requirements  of  the  drawing.  If 
one  will  make  a  few  such  lay-outs,  it  will  soon  be  clear  to  him 
that  there  are  always  a  number  of  dimensions  that  should  be 
given  without  tolerances  if  drawings  are  to  be  kept  consistent 
and  intelligible. 

Information  on  Component  Drawings.  It  must  be  realized  at 
the  start  that  it  is  impossible  in  every  case  to  give  on  one  com- 
ponent drawing  all  the  dimensions  that  are  needed  to  construct 
the  patterns,  tools,  gages,  and  other  manufacturing  equipment, 
without  introducing  many  inconsistencies.  Certain  dimensions 
could  be  correct  if  one  set  of  holding  points  and  one  series  of 
operations  were  to  be  used,  but  would  be  incorrect  under  differ- 
ent conditions.  If  the  component  drawings  are  made  so  that 
they  represent  the  proper  completed  conditions  —  call  them 
inspection  gage  requirements  if  you  will  —  the  end  in  view  is 
attained.  Any  figures  that  the  shop  desires  to  use  are  correct 
if  they  insure  this  result. 

It  is  impossible  to  amplify  this  point  without  entering  into 
a  prolonged  discussion  of  the  effect  of  using  different  holding 
or  registering  points  in  the  manufacturing  processes.  Yet  it 


8  INTERCHANGEABLE   MANUFACTURING 

may  be  of  interest  to  know  that  several  manufacturing  plants 
solve  this  problem  by  adding  operation  drawings,  which  give 
only  the  specific  dimensions  required  at  a  particular  operation. 
Some  of  the  dimensions  are  duplicates  of  those  on  the  component 
drawing,  while  others  are  computed  to  serve  their  restricted 
purpose.  This  proves  an  effective  means  of  recording  additional 
information  required  in  the  manufacturing  departments,  which 
cannot  be  put  on  component  drawings  without  danger  of  misuse. 

After  production  is  well  under  way,  the  component  drawings 
have  served  their  first  purpose.  In  the  meantime,  the  actual 
manufacturing  operations  have  made  available  a  store  of  new 
information  regarding  the  proper  conditions  to  be  maintained. 
It  should  be  the  second  function  of  the  component  drawings  to 
record  as  much  of  this  information  as  possible.  Conflicting 
information  or  misinformation  should  be  eliminated  at  the  same 
time;  in  short,  the  drawings  should  be  revised  to  agree  with 
actual  conditions  and  requirements.  It  has  been  a  great  fault 
in  the  past  to  neglect  this  second  function  almost  entirely.  It  is 
a  difficult  task  to  make  the  component  drawings  represent  from 
the  first  conditions  that  must  be  maintained.  In  time,  the 
shop  will  discover  many  of  them,  often  after  bitter  experience, 
even  though  they  have  been  omitted  from  the  component  draw- 
ings. Frequently,  however,  it  happens  that  this  information 
does  not  make  its  way  back  to  the  office,  but  is  retained  by  the 
shop  men  among  themselves.  Often  this  is  the  fault  of  the 
office,  which  is  prone  to  consider  such  information  as  criticism, 
so  that  the  shop,  after  a  few  rebuffs,  makes  no  further  attempt 
to  pass  it  along.  It  is  most  essential,  however,  that  such  in- 
formation be  recorded  in  permanent  form,  not  only  because  of 
its  value  to  the  work  in  hand,  but  also  because  of  its  helpful 
application  to  new  work  in  the  future. 

Dimensioning  of  Component  Drawings.  The  problem  of  the 
proper  dimensioning  of  component  drawings  is  strictly  a  mathe- 
matical one.  There  are  a  few  basic  principles  in  regard  to  it 
as  fixed  and  simple  as  Newton's  three  laws  of  motion,  but  even 
more  difficult  at  times  to  apply  correctly.  When  either  of  the 
two  following  principles  is  violated,  trouble  will  inevitably  follow: 


INTERCHANGEABLE    MANUFACTURING  9 

1.  In  interchangeable  manufacturing,  there  is  but  one  dimen- 
sion (or  group  of  dimensions)  in  the  same  straight  line  that  can 
be   controlled  within  fixed   tolerances.     That  is   the   distance 
between  the  cutting  surface  of  the  tool  and  the  locating  or 
registering  surface  of  the  part  being  machined.     Hence,  it  is 
incorrect  to  locate  any  point  or  surface  with  tolerances  from  more 
than  one  point  in  the  same  straight  line. 

2.  Dimensions  should  be  given  between  those  points  which 
it  is  essential  to  hold  in  a  specific  relation  to  each  other.    The 
majority  of  dimensions,  however,  are  relatively  unimportant 
in  this  respect.    It  is  good  practice  to  establish  locating  points 
in  each  plane,  and  to  give,  as  far  as  possible,  all  such  dimensions 
from  these  common  locating  points. 

There  are  also  a  few  other  general  principles  which  it  is  good 
practice  to  follow.  Although  violations  of  them  are  not  errors 
in  themselves,  they  lead  to  many  unnecessary  errors.  In  all  of 
this  work  it  must  be  realized  that  it  is  impossible  to  create 
anything  that  is  altogether  fool-proof;  the  best  that  can  be 
expected  is  to  make  conditions  such  that  little  or  no  excuse 
remains  for  making  a  mistake.  The  three  following  principles 
are  of  this  order: 

1.  The  basic  dimensions  given  on  component  drawings  for  / 
interchangeable   parts   should   be   the   maximum   metal   sizes, 
except  for  force  fits  and  other  unusual  conditions.    The  direct  1 
comparison  of  the  basic  sizes  should  check  the  " danger  zone"  I 
or  the  minimum  clearance  conditions  in  most  cases.    It  is  evident  ! 
that  these  sizes  are  the  most  important  ones,  as  they  control 
the  interchangeability.     They  should  be  the  first  determined 
and,  once  established,  they  should  remain  fixed  if  the  mechanism 
functions  properly  and  the  design  is  unchanged.    The  direction 
of  the  tolerances,  then,  would  be  such  as  would  increase  this 
clearance.     For  force  fits,  such  as  taper  keys,  etc.,  the  basic 
dimensions  should  be  those  which  determine  the  minimum  inter- 
ference (which  is  the  " danger  zone"  in  this  case)  and  the  direc- 
tion of  the  tolerances  for  this  class  of  work  should  be  such  as 
would  increase  this  interference. 

2.  Dimensions  should  not  be  duplicated  between  the  same 


10  INTERCHANGEABLE   MANUFACTURING 

points.  The  duplication  of  dimensions  causes  much  needless 
trouble,  due  to  changes  being  made  in  one  place  and  not  in  the 
others.  It  causes  less  trouble  to  search  a  drawing  to  find  dimen- 
sions than  it  does  to  have  them  duplicated  and,  though  more 
readily  found,  inconsistent. 

3.  As  far  as  possible,  the  dimensions  on  companion  parts 
should  be  given  from  the  same  relative  locations.  This  pro- 
cedure assists  in  detecting  interferences  and  other  improper 
conditions. 

If  careful  thought  is  given  to  these  component  drawings, 
much  time  and  effort  will  be  saved  later  in  the  shop.  If  they 
are  neglected,  all  the  future  work  will  suffer.  A  large  percentage 
of.  the  mistakes  made  in  the  manufacturing  departments  may 
be  traced  back  to  improper  component  drawings. 

Specifications  for  Interchangeable  Manufacturing.  The  in- 
formation that  can  be  included  on  component  drawings,  except 
in  the  case  of  a  very  simple  or  familiar  mechanism,  is  seldom 
sufficient  in  itself  to  enable  the  manufacturer  to  proceed  in- 
telligently with  a  new  product.  It  is  desirable  that  he  know  the 
particular  purpose  for  which  the  mechanism  is  to  be  made. 
The  better  he  is  informed  on  this  subject,  the  greater  service 
he  can  render  in  promoting  its  economical  manufacture  and 
future  development.  Specifications  are  supposed  to  supplement 
the  drawings  by  giving  all  the  needed  additional  information 
which  has  no  place  on  the  drawings.  I  say  "supposed"  because 
it  is  only  in  rare  cases  that  the  specifications  commonly  en- 
countered give  all  the  desirable  information.  They  usually 
deal  with  only  the  most  exacting  requirements  and  make  no 
mention  of  the  others,  thus  establishing  a  severe  precedent  for 
the  solution  of  all  questions  in  regard  to  the  requirements, 
important  and  unimportant.  They  seldom  indicate  the  essential 
object  in  view,  namely,  the  economical  production  of  mechanisms 
which  will  function  satisfactorily. 

Specifications  should  state  the  end  to  be  accomplished,  and 
should  give  all  possible  information  to  assist  in  the  attaining  of 
that  end.  Any  unusual  conditions  should  be  explained  in  detail. 
All  exacting  requirements  should  be  specified  with  the  reasons 


INTERCHANGEABLE   MANUFACTURING  II 

for  the  same,  including  requirements  of  functioning  and  of 
materials  to  be  used.  But  they  should  not  stop  here.  The  less 
exacting  conditions  should  be  noted  also.  If  a  certain  material 
is  specified,  and  the  chief  consideration  is  economy,  it  should 
be  so  stated,  with  the  substitution  allowed.  The  material  that 
might  be  most  economical  under  one  set  of  conditions  might  be 
otherwise  under  different  circumstances.  Parts  which  are 
detailed  on  the  drawings  but  for  which  commercial  articles  can 
be  substituted  should  be  so  designated.  The  specifications 
should  list  those  parts  which  must  be  made  interchangeable  and 
those  which  need  not  be.  A  description  of  the  tests  for  materials, 
for  physical  dimensions,  and  for  functioning  should  be  included. 
In  fact,  any  information  that  will  assist  in  the  manufacture  of 
the  product  should  be  given.  Some  of  it  will  specify  the  results 
to  be  obtained;  more  of  it  should  be  information  to  assist  the 
manufacturer,  not  hard  and  fast  rules  which  he  must  follow 
regardless  of  consequences. 

Such  specifications  would  undoubtedly  be  far  from  com- 
plete at  first.  Provision  should  be  made  to  keep  them  abreast 
of  the  actual  progress  of  the  work.  The  shop  should  use  them 
as  a  place  to  record  as  much  of  the  experience  gained  as  possible. 
If  certain  methods  have  been  found  unsatisfactory,  here  is  an 
ideal  place  to  record  the  fact,  and  perhaps  save  a  duplication 
of  the  mistake  in  the  future.  If  other  methods  have  proved 
satisfactory,  they,  too,  should  by  all  means  be  recorded.  In 
fact,  specifications  of  this  kind,  although  they  would  in  time 
become  voluminous,  would  be  a  history  of  a  mechanism  and 
furnish  valuable  data  to  assist  in  developing  new  mechanisms. 

Written  specifications  are  held  in  low  esteem  by  the  majority 
of  manufacturers.  They  do  without  written  specifications  for 
their  own  products,  and  when  obliged  to  meet  them  for  contract 
work,  find  them  an  additional  annoyance  instead  of  a  help. 
This  is  due,  in  large  measure,  to  the  fact  that  this  subject,  as 
also  the  matter  of  tolerances,  has  been  regarded  as  an  end  in 
itself  instead  of  as  a  means  to  a  larger  end.  Manufacturers  do 
have  specifications,  although  they  are  seldom  called  by  that 
name  and  are  seldom  written  or  grouped  together  for  ready 


12  INTERCHANGEABLE   MANUFACTURING 

reference.  Some  of  them  may  be  found  in  the  cost  and  pro- 
duction records,  some  in  the  shop  correspondence,  but  most 
of  them  are  carried  in  the  memories  of  the  foremen  and  older 
employes  who  maintain  the  traditions  of  the  shop. 

Gages  for  Checking  Results.  Thus  far  those  elements  which 
form  the  groundwork  for  actual  manufacturing  operations 
have  been  discussed.  The  manufacturing  design  has  been 
developed;  it  has  been  tested  with  the  manufacturing  model; 
the  first  guess  as  to  the  proper  manufacturing  tolerances  has 
been  made;  all  suitable  and  available  information  has  been 
recorded  on  the  component  drawings;  and  the  specifications 
to  supplement  these  drawings  have  been  partially  developed 
by  recording  there  all  further  information  available  that  will 
assist  in  accomplishing  the  main  purpose,  namely,  to  produce 
satisfactory  mechanisms  as  economically  as  possible.  The 
means  of  carrying  on  the  work  of  actual  production  and  the 
facilities  that  should  be  provided  for  checking  the  results,  must 
now  be  considered. 

There  are  two  important  reasons  for  inspecting  the  product 
during  manufacture:  First,  spoiled  parts  must  be  eliminated 
as  soon  as  possible  to  save  the  expenditure  of  useless  effort  on 
unserviceable  pieces.  Second,  the  completed  components  must 
be  checked  before  assembly  to  eliminate  the  unserviceable  parts 
and  thus  insure  the  proper  functioning  of  the  mechanism.  For 
these  purposes,  gages  are  extensively  employed. 

A  gage  should  be  provided  whenever  its  use  is  more  economical 
than  the  use  of  standard  measuring  instruments.  For  example, 
if  the  total  production  of  a  certain  mechanism  amounts  to  about 
a  dozen  units,  it  is  extravagance  to  provide  special  gages.  On 
the  other  hand,  if  this  production  amounts  to  several  thousand 
units,  a  complete  set  of  gages  is  both  desirable  and  necessary. 
The  extent  to  which  gages  are  necessary,  therefore,  depends  in 
great  measure  upon  the  amount  of  the  total  production.  Further- 
more, gages  should  be  provided  to  check  only  those  conditions 
which  it  is  essential  to  maintain.  The  nature  and  extent  of  the 
gages  required  depend  upon  the  manufacturing  conditions. 
In  many  cases,  a  check  on  one  or  two  points  is  sufficient  to  detect 


INTERCHANGEABLE   MANUFACTURING  13 

any  unsatisfactory  results.  Under  varying  manufacturing  con- 
ditions different  faults  must  be  guarded  against.  Gages  are 
a  preventive  and  not  a  cure.  The  point  to  be  emphasized  is  that 
they  should  be  provided  whenever  their  addition  will  result  in 
the  production  of  more  or  better  components  with  a  total  ex- 
penditure of  the  same  or  less  effort. 

Main  Classes  of  Gages.  There  are  two  kinds  of  gages  to  con- 
sider, which,  for  want  of  better  terms,  will  be  called  limit  gages 
and  functional  gages.  A  limit  gage  is  one  that  checks  a  specified 
dimension  to  specified  tolerances.  A  functional  gage  is  one  that 
checks  the  relationship  of  several  dimensions  to  insure  the 
proper  functioning  of  the  assembled  mechanism.  As  with  other 
manufacturing  equipment,  the  exact  design  of  a  gage  is  unim- 
portant, if  it  fulfils  its  purpose  simply  and  efficiently. 

The  degree  of  accuracy  or  precision  required  depends  upon 
the  extent  of  the  tolerances.  In  all  cases,  on  limit  gages,  the 
variation  must  be  inside  the  established  limits  of  the  component. 
The  dimensions  given  on  component  drawings  are  limit  gage 
sizes.  For  example,  the  limits  given  for  the  diameter  of  a  stud 
should  be  interpreted  to  mean  that  such  diameter  must  be  made 
to  satisfy  ring  or  snap  gages  of  the  sizes  specified. 

As  yet  master  gages,  or  reference  gages,  as  they  are  variously 
called,  have  not  been  touched  upon.  A  master  is  a  physical 
standard  of  size  or  form  used  for  reference  purposes.  It  is  needed 
only  where  the  degree  of  precision  required  is  so  exacting  that 
the  errors  inherent  in  direct  measurements  with  standard  measur- 
ing instruments  will  be  great  enough  to  prevent  the  proper 
functioning  of  the  product.  If  a  manufacturing  model  is  care- 
fully developed,  few,  if  any,  masters  will  be  required.  For 
simple  dimensions  of  length,  it  is  usually  sufficient  to  establish 
reference  pieces  of,  say,  tenth-inch  units.  For  important 
functional  contours,  masters  are  essential. 

Test  pieces  for  individual  gages  are  necessary  only  when  the 
amount  of  gage  checking  is  so  great  that  too  much  time  is  con- 
sumed by  using  standard  measuring  instruments,  or  when  no 
skilled  labor  is  available  for  this  checking.  Test  pieces  are 
therefore  desirable  for  checking  complicated  profile  and  fixture 


14  INTERCHANGEABLE    MANUFACTURING 

gages  that  receive  hard  usage,  but  they  are  seldom  necessary 
for  plain  plug,  ring,  and  snap  gages. 

Manufacturing  Equipment.  Suitable  tools  and  equipment  with 
which  to  manufacture  a  product  must  also  be  provided.  The 
first  logical  step  to  this  end  is  to  make  operation  lists,  planning 
in  detail  the  successive  operations,  and  specifying  the  type  of 
machine,  fixture,  tool,  and  gage  required.  These  operation  lists 
are  an  integral  part  of  the  specifications,  subject,  of  course,  to 
such  modifications  as  are  found  necessary.  Of  the  machines 
themselves  but  little  mention  need  be  made  at  this  time.  Stand- 
ard machine  tools  are  now  on  the  market  for  making  almost 
every  variety  of  machining  cut.  Special  machines  are  required 
only  for  very  unusual  operations  or  for  extremely  large  pro- 
ductions where  many  automatic  operations  are  performed. 

The  design  of  the  fixture  and  the  tool  depends  to  a  great 
extent  upon  the  design  of  the  piece  to  be  machined.  Great 
care  should  be  taken  to  maintain  the  same  locating  or  register- 
ing points  in  the  fixtures  as  are  used  for  the  gages.  The  ideal 
condition  is  to  have  the  registering  points  for  both  fixtures  and 
gages  identical  with  the  points  on  the  component  drawings 
from  which  the  surfaces  in  question  are  dimensioned.  After 
the  equipment  is  complete,  the  component  drawings  should  be 
checked  and  revised  T^here  necessary  to  obtain  this  result. 

Another  factor  which  must  be  considered  in  the  design  of  the 
equipment  is  the  required  rate  of  production.  In  the  case  of  a 
small  output,  the  cost  of  the  equipment  amounts  to  a  large 
percentage  of  the  total  cost  of  production.  As  the  output  in- 
creases, the  proportionate  cost  of  the  equipment  decreases, 
thus  making  it  desirable  to  refine  this  equipment,  if  by  so  doing 
the  production  can  be  increased  with  the  expenditure  of  less 
productive  effort.  Here,  as  elsewhere,  it  is  a  question  of  balanc- 
ing the  cost  of  one  item  against  that  of  another  and  of  selecting 
the  most  economical  combination. 

In  most  cases,  except  with  some  automatic  machines  or  on 
very  large  work,  the  operator  spends  more  time  in  handling  the 
work  than  the  machine  takes  to  perform  the  machining  operation. 
Therefore,  whenever  the  rate  of  production  is  high  enough  to  make 


INTERCHANGEABLE   MANUFACTURING  15 

it  economical,  the  fixtures  should  be  made  for  rapid  operation, 
even  though  this  greatly  increases  the  initial  cost  of  equiqment. 

Production  Problems.  The  actual  production  consists  of  taking 
the  raw  material  and  passing  it  through  the  equipment  until  it 
emerges  as  a  finished  component.  The  production  problems 
are  many  and  varied.  Any  part  of  the  preceding  work  which 
has  been  slighted  or  left  undone  must  be  completed  here  in 
addition  to  the  many  tasks  which  are  involved  in  the  production 
itself.  The  greatest  problem  involved  in  production  is  that 
most  uncertain  factor  —  human  nature.  The  present  tendency 
is  to  provide  equipment  that  can  be  operated  by  semi-skilled 
labor.  Equipment,  however,  cannot  be  made  altogether  fool- 
proof. As  noted  before,  the  best  that  can  be  done  is  to  arrange 
matters  so  that  little  or  no  excuse  remains  for  making  mistakes. 

People  thoughtlessly  speak  of  unskilled  labor.  The  more  this 
problem  is  studied,  the  more  it  is  realized  that  there  is  no  place 
in  interchangeable  manufacturing  for  such  assistance.  That 
is,  there  is  no  task  so  elementary  but  that  better  and  more 
economical  results  can  be  obtained  by  a  certain  degree  of  train- 
ing or  skill  in  the  operator.  An  attempt  is  made  to  subdivide 
productive  operations  into  the  most  elementary  tasks  so  that 
labor  can  be  readily  trained  to  perform  them  satisfactorily. 
Each  manufacturer  is  forced  to  train  the  majority  of  his  own 
operators.  Naturally,  then,  the  shorter  the  time  required  for 
this  training,  the  sooner  the  results  will  show  in  the  production. 
On  the  other  hand,  the  less  skill  required  of  the  operator,  the 
more  elaborate  and  complete  the  equipment  must  be.  The 
amount  of  supervision  required  for  both  operators  and  equip- 
ment is  also  greatly  increased,  in  both  quantity  and  quality. 

In  any  case,  the  better  the  training  that  these  operators 
receive,  the  higher  is  the  quality  of  the  work  produced.  And 
the  matter  of  honest,  serviceable  quality  as  distinguished  from 
mere  appearance  is  more  appreciated  than  formerly.  The 
operator  should  be  taught  to  maintain  the  established  tolerances. 
If  the  specified  tolerances  prove  too  severe  in  practice  for  eco- 
nomical production,  they  should  be  corrected,  provided  the 
functional  requirements  of  the  mechanism  will  permit.  If  they 


1 6  INTERCHANGEABLE   MANUFACTURING 

are  not  too  severe,  there  is  no  excuse  for  violating  them.  The 
practice  of  adhering  to  the  specified  tolerances  will  do  much  to 
promote  a  high  quality  of  product. 

Shop  Inspection  of  Product.  The  inspection  and  acceptance 
or  rejection  of  the  components  falls  logically  into  two  divisions. 
The  first  is  the  shop  inspection  which  is  made  while  the  material 
is  in  process  of  manufacture.  The  object  is  to  cull  out  defective 
work  as  soon  as  possible  and  also  to  detect  any  defects  in  the 
equipment  that  would  result  in  faulty  work.  If  the  percentage 
of  rejections  is  normal,  it  is  evident  that  the  requirements  speci- 
fied and  the  manufacturing  facilities  provided  are  satisfactdry. 
If  the  percentage  is  high,  it  is  evidence  of  improper  conditions 
somewhere  which  should  be  investigated,  and  the  trouble  should 
be  corrected  at  its  source.  Sometimes  an  error  occurs,  with  the 
result  that  the  requirements  are  exceeded  on  a  large  number 
of  parts.  Such  matters  should  be  investigated  and  settled 
according  to  their  merits.  If  the  pieces  will  be  serviceable  and 
can  be  completed  without  undue  cost,  the  factor  of  economy 
will  play  a  large  part  in  the  decision.  In  such  cases,  the  require- 
ments specified  should  not  be  changed  unless  it  is  evident  that 
such  a  change  will  result  in  an  economic  benefit  in  the  future. 
As  in  all  other  cases,  ultimate  economy  is  the  goal. 

Final  Inspection.  The  second  division  of  the  inspection  is  the 
final  examination  of  the  completed  parts.  The  object  of  this 
inspection  is  to  see  that  all  components  which  will  function 
properly  are  accepted  and  that  all  unserviceable  parts  are  re- 
jected. This  inspection  is  largely  governed  by  the  requirements 
of  the  component  drawings  —  often  represented  by  gages  —  and 
by  the  specifications.  It  is  therefore  most  important  that  the 
drawings  and  specifications  give  as  nearly  as  possible  the  limits 
of  parts  which  will  function  properly.  Yet,  as  has  been  already 
noted,  these  drawings  and  specifications  are  incomplete  at  the 
beginning,  and  probably  will  always  be  so,  to  a  certain  extent. 
Therefore,  a  rigid  adherence  to  the  letter  but  not  to  the  spirit 
of  the  drawings  and  specifications  is  unwise,  as  it  will  not  aid  in 
the  acceptance  of  all  serviceable  material,  nor  in  the  ultimate 
economy  of  manufacture.  In  addition  to  the  written  require- 


INTERCHANGEABLE    MANUFACTURING  iy 

ments,  inspectors  must  have  a  certain  amount  of  education  and 
experience  with  the  mechanisms  involved,  or  with  similar 
mechanisms;  otherwise  the  inspection  will  always  prove  a 
hindrance  to  the  main  purpose. 

The  characteristic  needed  for  a  successful  inspector  is  a 
judicial  mind.  Since  the  requirements  are  laws,  the  inspection 
should  equitably  enforce  them.  The  spirit  of  the  requirements 
should  be  enforced  in  those  cases  where  their  exact  expression 
is  incomplete.  If  the  essentials  are  always  specified  definitely 
and  completely,  it  will  be  a  fair  assumption  that  incompletely 
specified  conditions  are  relatively  unimportant.  Wherever 
possible,  the  requirements  should  be  revised  to  make  the  letter 
and  the  spirit  agree,  but  the  attempt  to  cover  every  minute  and 
unimportant  detail  will  prove  impossible  in  practice. 

The  functional  requirements  should  be  maintained  in  the 
final  inspection  strictly  according  to  the  specified  conditions. 
The  non-functional  requirements  should  be  handled  in  a  more 
judicial  manner,  each  case  being  decided  on  its  merits.  As  a 
matter  of  fact,  this  final  inspection  should  be  in  the  nature  of  a 
functional  inspection  only.  Little  attention  should  be  given 
here  to  the  non-essentials  other  than,  perhaps,  a  visual  inspection 
for  general  quality,  and  some  supervision  of  the  shop  inspection 
to  see  that  proper  precautions  are  taken  during  production  to 
insure  a  good  product.  In  all  cases,  the  main  effort  throughout 
the  work  should  be  to  establish,  define,  and  maintain  the  essential 
conditions  first,  letting  the  non-essentials  develop  in  practice. 
No  secret,  however,  should  be  made  of  the  fact  that  these  non- 
essentials  are  left  to  work  out  their  own  salvation. 

Test  of  Success.  The  final  and  complete  evidence  as  to  whether 
the  aim  has  been  accomplished  is  furnished  after  the  mechanisms 
have  been  assembled  and  tested.  If  the  total  costs  have  been 
reasonable  and  the  completed  mechanisms  assemble  properly 
and  perform  satisfactorily  all  the  required  functions,  it  is  con- 
clusive evidence  that  all  essentials  have  been  mastered.  On 
the  other  hand,  if  the  costs  are  excessive  or  if  the  mechanism 
fails  to  assemble  or  to  operate  properly  after  being  assembled, 
it  is  equally  conclusive  evidence  of  failure. 


CHAPTER  II 
TERMS    USED    IN    INTERCHANGEABLE    MANUFACTURING 

IN  order  to  describe  concisely  characteristics  peculiar  to 
interchangeable  manufacturing,  it  is  necessary  to  use  many 
words  and  phrases  in  an  arbitrary  sense.  Therefore,  to  avoid 
misunderstanding,  space  is  taken  here  to  define  several  of  the 
important  terms.  The  interpretation  of  these  terms  is  limited  to 
the  ideas  they  express  in  this  treatise. 

Interchangeability.  The  term  interchangeability,  as  used 
here,  refers  to  absolute  interchangeability.  In  this  sense,  inter- 
changeable parts  are  parts  that  are  so  made  that  they  can  be 
assembled  or  interchanged  after  final  inspection  without  machin- 
ing or  fitting,  and  any  possible  combination  of  these  parts  will 
assemble,  interchange,  and  function  properly.  To  insure  this 
end,  the  most  extreme  limits  permitted  must  be  constantly 
checked  against  each  other. 

Selective  Assembly.  Selective  assembly  refers  to  a  method 
of  manufacturing  similar  in  many  of  its  details  to  interchange- 
able manufacturing,  in  which  component  parts  are  sorted  and 
mated  according  to  size  and  assembled  or  interchanged  with 
little  or  no  machining.  Companion  parts  made  to  the  extreme 
limits  are  not  supposed  to  interchange.  For  instance,  a  maximum 
male  component  will  not  assemble  with  a  minimum  female 
part.  However,  the  maximum  male  and  female,  or  the  minimum 
male  and  female  must  interchange.  A  good  example  of  this 
method  of  assembling  is  found  in  the  production  of  ball  bearings. 
The  balls  are  sorted  into  groups,  according  to  their  size,  to  facili- 
tate the  assembly  of  any  bearing  with  balls  of  uniform  size. 
As  a  matter  of  fact,  nearly  every  so-called  interchangeable 
article  represents  a  combination  of  the  two  methods  of  quantity 
production  —  interchangeable  and  selective. 

18 


DEFINITIONS   OF   TERMS  1 9 

Function.  The  term  function  is  used  extensively  and  with 
various  shades  of  meaning.  The  word  itself  has  many  meanings. 
The  dictionary  gives  one  as  "fulfillment  or  discharge  of  a  set 
duty  or  requirement";  and  another  as  "that  mode  of  action 
or  operation  which  is  proper  to  any  structure,"  etc.  As  applied 
to  component  parts,  the  word  has  been  used  to  express  both 
these  meanings.  This  includes  all  requirements  of  interchange- 
ability  and  service  which  the  part  must  render  throughout  the 
normal  life  of  the  mechanism  of  which  it  forms  a  part.  The 
same  meaning  is  intended  when  it  is  applied  to  the  assembled 
mechanism.  The  functional  design  refers  specifically  to  the 
combination  of  mechanical  movements  required  to  make  the 
completed  mechanism  perform  its  specified  duties.  Functional 
gages  are  those  which  test  the  functional  operation  of  components 
without  strict  adherence  to  their  exact  physical  dimensions. 

Limit.  In  every  interchangeable  mechanism  there  are  certain 
maximum  and  minimum  sizes  for  each  part,  between  which  the 
parts  will  function  properly  in  conjunction  with  each  other 
and  outside  of  which  they  will  not.  These  sizes  are  the  absolute 
limits  of  the  parts.  The  established  limits  are  the  maximum 
and  minimum  dimensions  specified  on  the  component  drawings. 
The  established  limits  should  approach  as  closely  to  the  absolute 
limits  as  normal  manufacturing  conditions  require.  Limits 
established  without  regard  to  the  absolute  limits  result  either 
in  excessive  cost  of  manufacture  or  faulty  mechanisms  or  both. 
If  the  established  limits  are  much  more  severe  than  the  absolute 
limits,  needless  expense  is  incurred  in  manufacturing.  On  the 
other  hand,  if  the  established  limits  are  more  liberal  than  the 
absolute  limits,  unsatisfactory  mechanisms  will  be  produced. 

Tolerance.  Tolerance  is  the  amount  of  variation  permitted 
on  dimensions  or  surfaces.  The  tolerance  is  equal  to  the  differ- 
ence between  the  maximum  and  minimum  limits  of  any  specified 
dimension.  For  example,  if  the  maximum  limit  for  the  diameter 
of  a  shaft  was  2.000  inches  and  its  minimum  limit  was  1.990 
inches,  the  tolerance  for  this  diameter  would  be  o.oio  inch. 
By  determining  the  maximum  and  minimum  clearances  required 
on  operating  surfaces,  the  extent  of  these  tolerances  is  established. 


2O 


INTERCHANGEABLE   MANUFACTURING 


The  application  of  the  tolerances  to  the  basic  dimensions  fixes 
the  limits. 

Basic  and  Model  Size.  Obviously,  the  absolute  limits  of  the 
various  dimensions  and  surfaces  indicate  danger  points,  inas- 
much as  parts  made  beyond  these  limits  are  unserviceable.  A 
careful  analysis  of  a  mechanism  shows  that  one  of  these  danger 
points  is  more  sharply  defined  than  the  other.  For  example,  a 
certain  stud  must  always  assemble  into  a  certain  hole.  If  the 
stud  is  made  beyond  its  maximum  limit,  it  will  soon  be  tqo  large 
to  assemble.  If  it  is  made  beyond  its  minimum  limit,  it  will  be 
too  loose  or  too  weak  to  function.  The  absolute  maximum 


HOLE    1.250 +°.»  DIA. 


STUD   1.248  Zows"  DIA- 

\ 


T 


r€m 


i  i!   i  II 

VT'I'* 

it  1 


Machinery 


Fig.  1.     Graphic  Illustration  of  the  Meaning  of  the  Terms  Limit 
and  Tolerance 

limit  in  this  case  can  be  defined  within  a  range  of  o.ooi  inch, 
whereas  the  absolute  minimum  limit  cannot  be  defined  within 
a  range  of  at  least  0.004  mcn-  In  this  case  the  maximum  limit 
is  the  more  sharply  defined. 

The  basic  size  expressed  on  the  component  drawing  is  that 
limit  which  defines  the  more  vital  of  the  two  danger  points, 
while  the  tolerance  defines  the  other.  In  general,  the  basic 
dimension  of  a  male  surface  is  the  maximum  limit  which  re- 
quires a  minus  tolerance.  Similarly,  the  basic  dimension  of  a 
female  surface  is  the  minimum  limit  requiring  a  plus  tolerance, 


DEFINITIONS    OF   TERMS  21 

as  shown  in  Fig.  i.  There  are,  however,  dimensions  which  define 
neither  a  male  nor  a  female  surface.  Such  are  dimensions  for 
the  location  of  holes.  In  a  few  cases  of  this  kind,  a  variation  in 
one  direction  is  less  dangerous  than  a  variation  in  the  other. 
Under  these  conditions,  the  basic  dimension  represents  the  dan- 
ger point,  and  the  tolerance  permits  a  variation  only  in  the  less 
dangerous  direction.  At  other  times,  the  conditions  are  such 
that  any  variation  from  a  fixed  point  in  either  direction  is  equally 
dangerous.  In  such  a  case,  the  basic  size  represents  this  fixed 
point.  Tolerances,  when  given  on  the  component  drawing, 
extend  equally  in  both  directions. 

If  a  model  is  developed  as  a  standard  of  precision,  the  model 
parts  become  the  physical  representations  of  the  basic  sizes. 
In  other  words,  for  all  practical  purposes,  the  model  size  and  the 
basic  size  are  identical. 

Maximum  Metal  Size.  Maximum  metal  size  is  that  limit  at 
which  the  part  contains  the  maximum  amount  of  metal.  This 
would  be  the  maximum  male  limit  and  the  minimum  female 
limit.  In  many  cases,  a  careful  analysis  is  necessary  to  determine 
which  limit  represents  the  maximum  metal  conditions,  as  many 
dimensions  are  neither  male  nor  female.  In  other  cases,  such 
as  locations  of  holes,  there  are  neither  maximum  nor  minimum 
metal  conditions.  With  few  exceptions,  however,  the  maximum 
metal  sizes  are  also  the  basic  sizes. 

Minimum  Metal  Size.  Similarly,  the  minimum  metal  size 
is  that  limit  at  which  the  part  contains  the  minimum  amount 
of  metal.  This  is  the  minimum  male  limit  and  the  maximum 
female  limit,  when  the  dimensions  can  be  so  classified. 

Minimum  Clearance.  It  is  evident  that  there  must  be  a  definite 
amount  of  clearance  between  male  and  female  components 
which  operate  together.  The  minimum  clearance  should  be  as 
small  as  will  permit  the  ready  assembly  and  operation  of  the 
parts,  while  the  maximum  clearance  should  be  as  great  as  the 
functioning  of  the  mechanism  will  allow.  The  difference  between 
the  maximum  and  minimum  clearances  defines  the  extent  of  the 
tolerances.  On  companion  elementary  surfaces,  the  difference 
between  the  maximum  male  limit  and  the  minimum  female 


22 


INTERCHANGEABLE   MANUFACTURING 


limit  determines  the  minimum  clearance,  as  shown  in  Fig.  2. 
On  composite  surfaces,  careful  study  is  required  to  determine 
which  limit  should  be  used.  In  fact,  it  is  impossible  in  certain 
cases  to  have  the  minimum  clearance  conditions  at  all  points  at 
the  same  time.  In  general,  however,  the  comparison  of  the 
basic  sizes  of  companion  parts  gives  the  minimum  clearance 
conditions.  The  minimum  clearance  is  quite  commonly  known 
as  the  " allowance." 

Maximum  Clearance.  On  elementary  surfaces,  the  difference 
between  the  minimum  male  limits  and  the  maximum  female 
limits  establishes  the  maximum  clearances.  In  general,  the 


HOLE  1.250±°-^''DIA, 


6TUD  1.2483 


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see 

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1° 

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1 


Machinery 


Fig.  2. 


Graphic  Illustration  of  the  Meaning  of  the  Terms  Maximum 
and  Minimum  Clearance 


terms  maximum  or  minimum  clearance  refer  only  to  the  clear- 
ance between  surfaces  which  operate  together  or  within  close 
proximity  to  each  other.  When  surfaces  stand  well  clear  of  each 
other,  and  there  is  little  or  no  danger  of  interference,  as  between 
unfinished  forged  or  cast  surfaces,  the  matter  of  maximum  and 
minimum  clearance  plays  little  part  in  determining  the  toler- 
ances. 

Interference.  If  a  male  member  is  larger  than  a  female  mem- 
ber, it  is  obvious  that  there  will  be  interference  when  these  parts 
are  assembled  together.  Such  interference  is  required  where 


DEFINITIONS    OF   TERMS 


force  fits  are  specified.  If  interchangeable  parts  are  to  be  forced 
together,  this  interference  performs  a  similar  function  to  that 
of  clearance  on  operating  surfaces.  In  this  case,  the  minimum 
interference  establishes  the  danger  point.  This  means  that 
for  force  fits  the  basic  male  dimension  is  the  minimum  limit 
requiring  a  plus  tolerance,  while  the  basic  female  dimension  is 
the  maximum  limit  requiring  a  minus  tolerance.  (See  Fig.  3.) 
When  the  component  drawings  permit  an  interference  where 


STUD  1.252  ±n'onn''DlA. 


HOLE  1.250 +°'°°°»DIA. 


r 

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t  KJ 

fl 

I  m 


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Fig.  3.     Graphic  Illustration  of  the  Meaning  of  the  Terms  Maximum 
and  Minimum  Interference 


a  clearance  is  required,  they  are  wrong.  The  term  interference 
is  often  used  to  express  such  conditions  of  error. 

Operating  Surfaces.  The  term  operating  surface  is  used  to 
distinguish  the  working  surfaces  of  the  mechanism  from  the 
others.  It  is  clear  that  the  working  surfaces  are  the  essential 
ones;  all  others  are  present  only  because  of  the  necessity  of 
holding  the  mechanism  together.  Generally  speaking,  the  oper- 
ating surfaces  are  the  machined  surfaces,  while  the  others  often 
retain  their  original  forged  or  cast  finish.  The  operating  surfaces 
are  divided  into  two  classes,  which  are  designated  functional  and 
non-functional,  or  clearance,  surfaces. 

Functional  Surfaces.  The  functional  surfaces  are  those  oper- 
ating surfaces  which  control  the  functioning  of  the  mechanism, 


INTERCHANGEABLE   MANUFACTURING 


as  shown  in  Fig.  4.  These  must  naturally  be  held  to  the  closest 
limits.  Every  operating  part  of  a  mechanism  must  be  controlled 
in  operation  within  reasonably  close  limits  in  each  plane.  After 
these  functional  requirements  of  location  are  met,  all  other 
surfaces  should  have  as  large  clearances  as  possible,  unless  the 
factor  of  strength  is  the  controlling  one.  Those  surfaces  that 
affect  the  relative  location  of  the  operating  parts  in  operation 
are  the  functional  surfaces.  For  example,  the  surface  of  a  pad 
on  which  a  bracket  that  carries  operating  parts  is  fastened  is  a 


BREECH-RING 


CLEARANCE  SURFACES 


CLEARANCE  SURFACES 


BREECH-BLOCK 


DIRECTION  OF 

PRESSURE  ON 

BREECH-BLOCK 

WHEN  GUN 

IS  FIRED 


Machinery 


Fig.  4.    Illustration  showing  the  Meaning  of  the  Terms  Functional 
and  Clearance  Surfaces 

functional  surface;  whereas,  the  surface  of  a  pad  that  supports 
a  bracket  for  holding  wrenches  or  oil-cans  is  not. 

Clearance  Surfaces.  Clearance  surfaces  are  those  operating 
surfaces  which  are  not  functional  surfaces.  In  this  class  are 
surfaces  which  do  not  control  the  location  of  operating  members 
while  functioning,  but  which  either  prevent  them  from  being 
disassembled  or  locate  them  approximately  in  their  inactive 
position,  or  both. 

Atmospheric  Fits.  Atmospheric  fits,  as  the  name  implies, 
refers  to  those  surfaces  which,  under  all  conditions,  stand  entirely 
clear  of  any  other  operating  or  functional  members  of  the  mech- 
anism. Such  is  the  outside  of  a  machine  frame.  Many  surfaces 


DEFINITIONS    OF   TERMS 


on  operating  parts  are  themselves  also  atmospheric  fits.  With 
few  exceptions,  the  majority  of  the  surfaces  of  all  mechanisms 
are  atmospheric  fits. 

Elementary  Surfaces.  An  elementary  surface  is  one  which  is 
defined  with  a  single  dimension,  such  as  a  cylinder,  a  plane,  or  a 
sphere.  For  example,  a  reamed  hole  of  a  speicfied  depth  rep- 
resents two  elementary  surfaces.  The  diameter  defines  one 
and  the  depth  the  other.  Obviously,  most  surfaces  which  are 
not  elementary  in  themselves  are  a  combination  of  elementary 
surfaces.  In  so  far  as  such  surfaces  are  machined  and  measured 
according  to  their  elements,  they  are  considered  elementary 
surfaces.  When  the  combination  as  a  whole  is  measured  or 


y 


Y 


Machinery 


Fig.  5. 


Illustration  showing  the  Meaning  of  the  Terms  Elementary 
and  Composite  Surfaces 


machined,  and  a  variation  on  one  surface  affects  the  dimension 
of  another,  they  are  not  elementary  but  composite  surfaces. 

Composite  Surfaces.  Composite  surfaces  are  those  surfaces 
which  are  required  to  maintain  a  co-relation  which  cannot  be 
expressed  by  a  single  dimension.  For  example,  Fig.  5  shows  a 
yoke  end.  The  over-all  dimension  (2.500  inches)  controls 
elementary  surfaces.  The  dimension  of  the  slot  (0.750  inch)  and 
that  of  its  location  (0.875  inch)  also  define  elementary  surfaces 
when  used  independently.  If,  however,  surfaces  marked  A,  B, 
and  C  are  required  to  be  checked  concurrently,  these  elementary 


26  INTERCHANGEABLE   MANUFACTURING 

surfaces  become  composite.  Irregular  profiles,  the  co-relation 
of  several  holes  to  each  other,  tapered  surfaces,  thread  sizes 
and  forms,  the  contour  and  location  of  gear  teeth,  etc.,  are 
examples  of  more  complicated  composite  surfaces  than  the 
example  shown  in  Fig.  5. 

Compound  Tolerances.  A  compound  tolerance  refers  to  those 
conditions  where  the  established  tolerances  on  more  than  one 
dimension  determine  the  required  limits.  These  exist  in  con- 
junction with  the  dimensioning  of  composite  surfaces.  For 
example,  a  compound  tolerance  exists  in  establishing  the  location 
of  surface  C  in  Fig.  5  from  surface  A.  This  condition  of  com- 
pound tolerances  will  be  covered  in  greater  detail  in  a  subsequent 
chapter. 

Working  or  Register  Points.  The  working  or  register  points 
are  those  surfaces  that  are  employed  for  locating  the  parts  in  the 
jigs  and  fixtures  during  the  process  of  manufacture.  Sometimes 
important  functional  surfaces  are  used  for  this  purpose.  In 
other  cases,  for  parts  of  irregular  form,  special  lugs  are  provided 
to  serve  this  end.  These  are  removed  after  the  machining 
operations  are  complete.  Register  points  become  functional 
surfaces  when  they  are  employed  to  machine  other  functional 
surfaces.  As  few  locating  points  as  possible  should  be  estab- 
lished; this  practice  simplifies  the  design  of  the  gages  and  other 
equipment. 

Unit  Assembly.  Many  mechanisms  are  a  combination  of 
several  semi-independent  mechanisms  which  may  be  assembled 
and  tested  individually  before  they  are  assembled  together. 
This  is  known  as  unit  assembly;  it  is  of  particular  value  for  a 
plant  which  manufactures  a  varied  product  where  such  unit 
assemblies  are  interchangeable.  An  example  would  be  a  feed- 
box  that  could  be  used  on  several  types  of  machines.  This 
practice  enables  a  plant  to  obtain  the  benefits  of  quantity  pro- 
duction on  these  unit  assemblies  although  the  quantity  of  pro- 
duction on  any  one  type  of  machine  is  small. 

Precision.  There  are  two  characteristics  pertaining  to  the 
physical  dimensions  of  the  parts  manufactured.  For  purposes 
of  discussion,  they  will  be  called  precision  and  accuracy.  The 


DEFINITIONS    OF   TERMS  27 

two  are  often  considered  identical,  and  if  ideal  conditions  could 
be  maintained,  they  would  be  identical.  In  ordinary  manu- 
facturing practice,  however,  precision  alone  is  usually  obtained, 
and  precision  is  all  that  is  necessary  in  most  cases.  It  is  when 
several  factories  attempt  independently  to  produce  a  common 
interchangeable  product  that  accuracy  is  required.  The  degree 
of  precision  is  measured  by  the  amount  of  variation  that  exists 
between  duplicate  parts.  For  example,  a  reamed  hole  is  dimen- 
sioned as  0.500  inch  in  diameter.  In  manufacture,  a  product  is 
obtained  in  which  the  difference  between  the  largest  and  smallest 
holes  produced  does  not  exceed  0.005  inch.  The  degree  of  pre- 
cision in  this  case  would  be  0.005  mcn>  even  though  the  absolute 
size  of  the  largest  hole  was  0.508  inch. 

Accuracy.  The  accuracy  of  any  determination  is  measured 
by  its  limits  of  error  from  a  fixed  standard.  For  example,  a 
length  of  one  inch  is  to  be  measured.  We  will  assume,  for  the 
sake  of  argument,  that  the  measuring  instruments  are  absolute. 
This  length  measured  with  an  ordinary  steel  scale  gives  a  result 
correct  within  a  limit  of  error  of  about  o.oi  inch.  If  measured 
with  a  micrometer,  the  result  is  correct  within  a  limit  of  error 
of  about  0.005  incn-  If  measured  on  a  sensitive  measuring 
machine,  the  result  is  correct  within  a  limit  of  error  of  about 
o.ooooi  inch;  while  if  measured  by  optical  methods,  advantage 
being  taken  of  the  principle  of  interference  of  light  waves,  the 
result  is  correct  within  a  limit  of  error  of  o.oooooi  inch  or  less. 
It  may  be  of  interest  to  note  here  that  standards  of  length  have 
been  defined  by  the  Bureau  of  Standards  in  terms  of  light  waves. 
By  this  means,  an  absolute  standard  is  established,  since  the 
lengths  of  light  waves  are  absolute.  The  term  accuracy  implies 
a  comparison  with  a  fixed  standard.  In  the  example  given  to 
illustrate  precision,  the  limit  of  precision  is  0.005  inch,  while 
the  limit  of  accuracy  is  0.008  inch.  It  is  obvious  from  this  that 
it  is  more  difficult  to  maintain  a  limit  of  accuracy  of  o.ooi  inch 
than  it  is  to  maintain  a  limit  of  precision  of  the  same  amount. 

Component  Drawings.  Component  drawings  are  detailed 
drawings  of  the  component  parts  of  a  mechanism.  For  inter- 
changeable manufacturing,  these  drawings  show  the  completed 


28  INTERCHANGEABLE   MANUFACTURING 

dimensions  required  (or  inspection  gage  requirements)  of  the 
parts.  These  include  all  tolerances  and  any  sub-assemblies 
that  may  be  necessary  to  assist  in  their  proper  interpretation. 

Operation  Drawings.  An  operation  drawing  is  a  detailed 
drawing  or  sketch  which  gives  the  dimensions  required  on  an 
individual  machining  operation.  These  drawings  are  used  to 
record  much  supplementary  information  that  might  be  confusing 
or  misleading  on  the  component  drawings,  such  as  allowances 
for  finishing  cuts  and  grinding  operations,  etc.  • 


CHAPTER  III 

MACHINE   DESIGN    IN   INTERCHANGEABLE 
MANUFACTURING 

THE  improvement  in  manufacturing  methods  and  facilities 
during  the  past  forty  or  fifty  years  has  been  very  rapid.  Quantity 
production  is  now  the  order  of  the  day.  New  problems  have 
arisen  and  old  ones  must  be  constantly  re-solved  to  meet  the 
situations  thus  created.  Manufacturing  on  an  interchangeable 
basis  has  been  a  direct  development  of  this  advance.  The  pur- 
pose of  the  present  chapter  is  to  discuss  the  effects  of  this  develop- 
ment on  the  design  of  a  machine  or  device,  and  to  emphasize 
those  practices  which  will  promote  economical  manufacture  on 
an  interchangeable  basis. 

In  early  days,  the  design  of  a  new  mechanism  existed  only  in 
the  mind  of  the  mechanic  engaged  in  its  construction.  It  was 
made  piece  by  piece,  each  detail  taking  definite  shape  as  it  was 
constructed.  The  original  mechanism  was  completed,  tested, 
and  corrected  or  rebuilt  before  the  design  was  finished.  Dupli- 
cate mechanisms  that  might  be  constructed  were  patterned 
after  the  original,  and  modified  or  improved  as  suited  the  ideas 
of  the  mechanics  who  performed  the  actual  work.  Needless 
to  say,  interchangeability  and  quantity  production  were  non- 
existent factors. 

Sketches  and  drawings  were  next  employed  to  express  the 
ideas  of  the  inventor,  but  little  attempt  was  made  to  indicate 
more  than  the  general  idea  and  construction.  The  details  of 
the  design  and  the  dimensions  of  the  individual  parts  were 
matters  for  the  workmen  to  decide.  A  competent  mechanic 
was  required  to  determine  these  for  himself.  It  was  part  of  his 
training.  Details  and  dimensions,  more  or  less  complete  and 
consistent,  made  their  appearance  on  the  drawings  later.  Errors 
and  omissions  were  of  little  moment,  as  the  mechanic  who  worked 
to  the  drawings  expected  to  select  and  use  the  proper  infor- 

29 


30  INTERCHANGEABLE   MANUFACTURING 

mation  given  and  to  ignore  the  incorrect,  supplying  all  omissions 
from  his  own  store  of  mechanical  knowledge  and  experience. 
The  quantity  of  production  was  small.  Little  or  no  importance 
was  placed  on  interchangeability.  A  few  workmen  thoroughly 
acquainted  with  the  requirements  of  the  mechanism  or  with  the 
intentions  of  the  designer  performed  all  the  actual  work  of  con- 
struction. Under  these  conditions,  functional  drawings,  which 
make  no  pretense  of  giving  more  than  the  general  construction 
or  combination  of  mechanical  movements  and-  the  general  out- 
line of  the  detailed  parts,  are  sufficient. 

Function  of  Design.  Under  present  manufacturing  conditions, 
with  productive  operations  subdivided  into  elementary  tasks, 
with  productive  labor  trained  along  specialized  lines,  with  pro- 
ductive equipment  specialized  and  more  nearly  complete,  with 
the  rate  of  production  greatly  increased,  with  larger  organiza- 
tions in  which  but  few  individuals  are  thoroughly  conversant 
with  all  the  detailed  requirements  of  the  mechanism,  the  design 
must  cover  a  wider  field  and  be  much  more  comprehensive  and 
accurate.  In  addition  to  expressing  the  ideas  of  the  inventor, 
it  must  supply  most  of  the  knowledge  and  experience  formerly 
brought  to  this  work  by  the  mechanic. 

We  now  have,  therefore,  two  types  of  designing  to  consider, 
which  we  will  call  the  functional  design  and  the  manufacturing 
design.  The  manufacturing  design  is  a  detailed  development 
of  the  functional  design.  It  corrects  and  modifies  the  functional 
design  where  necessary,  to  facilitate  the  economical  production 
of  the  mechanism,  giving  as  much  as  possible  of  the  information 
previously  supplied  by  the  workman.  It  is  evident  that  the 
manufacturing  design  will  always  be  incomplete  to  a  certain 
extent.  Suitable  provision  for  its  modification  must  be  made  to 
obtain  the  advantage  of  the  new  and  improved  methods  of  manu- 
facture which  are  constantly  developed.  Changes,  however, 
in  proved  manufacturing  designs  should  be  avoided  when  pos- 
sible. As  much  or  greater  care  should  be  taken  in  adopting 
changes  as  is  exercised  in  establishing  the  original  manufacturing 
design.  After  equipment  has  been  completed,  changes  are  very 
costly.  A  change  which  might  be  justified  in  the  early  stages  of 


MACHINE  DESIGN  31 

work  often  costs  more  than  it  is  worth  in  the  later  stages.  This 
makes  it  of  the  utmost  importance  that  great  care  be  exercised 
in  the  development  of  the  original  manufacturing  design  of  a  new 
commodity  which  is  to  be  manufactured  in  large  quantities  on 
an  interchangeable  basis. 

Classes  of  Design.  For  the  construction  of  a  small  number  of 
special  machines,  or  tools  and  fixtures,  which  are  built  in  a 
general  machine  shop  or  tool-room,  the  functional  design  is 
all  that  is  required.  The  number  of  men  engaged  in  the  produc- 
tion is  small,  their  training  is  general,  and  the  requirements  of 
the  mechanisms  can  be  explained  to  them  personally  by  the 
designer  as  questions  arise;  therefore,  the  additional  expense 
of  a  manufacturing  design  is  not  justified.  However,  in  the 
manufacture  of  a  large  quantity  of  any  article,  particularly 
if  interchangeability  is  sought,  a  complete  manufacturing  design 
is  necessary.  True,  this  design  will  work  itself  out  in  practice 
in  the  course  of  time,  but  this  is  a  very  slow  and  expensive 
method.  It  means  that  experimental  work  on  a  large  scale  is 
carried  on,  whereas  it  can  be  done  on  a  smaller  scale  with  better 
and  speedier  results.  Furthermore,  this  method  results  in  con- 
tinual alterations  in  the  equipment  and  a  loss  of  interchange- 
ability.  However,  this  chapter  is  not  concerned  with  designs  of 
special  mechanisms,  tools,  fixtures,  etc.;  attention  will  here 
be  given  to  the  requirements  of  designing  as  applied  to  the  manu- 
facture of  a  product  in  large  quantities. 

In  both  types  of  designing,  the  end  in  view  is  the  same  as  far 
as  the  functioning  of  the  mechanism  is  concerned.  This  is  to 
develop  a  product  capable  of  performing  certain  results  which 
will  fill  or  create  a  public  demand  for  itself.  The  means  of  at- 
taining this  are  governed  by  various  considerations.  For  the 
functional  design,  any  solution  is  satisfactory.  As  regards  the 
manufacturing  design,  the  methods  adopted  must  result  in 
ultimate  economy.  Also,  the  manufacturing  design  must  re- 
semble the  functional  to  such  an  extent  that  all  patents  will  be 
retained,  while  those  of  competitors  must  not  be  infringed. 
This  is  one  of  the  commercial  difficulties  that,  at  times,  prevents 
the  true  economic  development  of  a  commodity. 


32  INTERCHANGEABLE   MANUFACTURING 

It  is  plain  that  the  manufacturing  designer  must  take  into 
consideration  every  circumstance  involved  in  the  production 
of  the  commodity.  To  be  successful,  he  must  work  in  close 
cooperation  with  all  who  will  be  engaged  in  the  development  and 
operation  of  the  manufacturing  equipment.  This  will  include 
the  tool  designers,  and  the  superintendents  and  foremen  of  the 
various  manufacturing  and  assembling  departments.  In  general, 
there  is  too  much  detail  involved  for  any  one  person  to  carry 
it  alone  to  a  successful  completion. 

Simplifying  Design.  When  considering  the  manufacture  of  a 
new  product,  one  of  two  conditions  usually  obtains.  Either 
it  is  to  be  produced  in  an  established  plant  with  an  existing 
variety  of  manufacturing  equipment,  or  a  new  plant  must  be 
created.  In  the  first  case,  the  designer  must  be  familiar  with  the 
available  equipment  and  must  modify  the  functional  design 
so  as  to  utilize  these  facilities  to  the  best  advantage.  In  the 
second  case,  he  is  not  restricted  to  the  use  of  any  specified  equip- 
ment. In  either  case,  unless  the  volume  of  production  is  to  be 
extremely  large  with  many  automatic  operations,  every  effort 
must  be  made  to  reduce  the  machined  surfaces  of  the  various  com- 
ponents to  simple,  elementary  surfaces  which  can  be  readily 
machined  on  standard  machine  tools  with  simple,  rugged,  and 
inexpensive  tools,  jigs,  and  fixtures.  If,  in  the  manufacturing 
design,  the  component  parts  are  thus  simplified,  a  further 
advantage  is  gained.  The  productive  operations  on  these  parts 
are  resolved  into  simple,  elementary  tasks,  and  this  simplifies 
the  problem  of  securing  and  training  the  necessary  productive 
labor.  Simplicity  is  a  primary  source  of  economy.  The  number 
of  machining  operations  is  reduced  and  the  direct  labor  cost 
thereby  lowered.  The  amount  of  time  that  raw  material  is 
tied  up  in  process  of  manufacture  is  reduced  and  quicker  re- 
turns are  secured  on  the  money  invested  in  direct  labor  and 
materials.  The  many  other  economies  resulting  from  simplicity 
in  design,  such  as  lower  equipment  and  maintenance  costs  are 
obvious. 

Factors  Governing  Choice  of  Materials.  Those  responsible 
for  the  manufacturing  design  must  pay  close  attention  to  the 


MACHINE   DESIGN  33 

character  of  the  materials  they  specify  for  the  individual  com- 
ponents. Ultimate  economy  is  the  desired  end.  This  is  affected 
by  many  different  and  sometimes  opposing  factors. 

Cost.  The  first  cost  of  the  material  is  one  of  these.  When 
several  thousand  duplicate  mechanisms  are  manufactured,  the 
slightest  saving  in  the  cost  of  direct  materials  is  multiplied  over 
and  over  again  in  the  course  of  time.  As  many  parts  as  possible 
should  be  made  of  the  same  size  and  kind  of  material.  This 
permits  purchasing  in  larger  quantities  and  reduces  the  gross 
amount  of  raw  material  carried  in  the  store-room.  As  far  as 
possible,  this  material  should  be  of  standard  sizes  and  forms  that 
can  be  purchased  in  the  open  market  at  the  lowest  prices. 

Source  of  Supply.  Due  consideration  must  be  given  to  the 
possible  sources  of  supply  for  the  materials  specified.  It  is  a 
serious  matter  when  production  is  held  up  because  of  lack  of 
material  which  has  a  limited  or  uncertain  source  of  supply. 
Every  effort  must  be  put  forth,  in  making  the  manufacturing 
design,  to  specify  materials  which  are  readily  secured. 

Machining  Qualities.  The  actual  economy  of  low-priced  mate- 
rial is  governed  by  the  ease  with  which  it  can  be  machined.  If 
a  part  requires  many  machining  operations,  a  low  initial  cost 
for  material  is  often  overbalanced  by  the  greater  cost  of  manu- 
facture. Therefore,  if  a  more  expensive  material  can  be  machined 
at  a  lower  cost,  ultimate  economy  dictates  its  purchase.  For 
this  reason,  the  use  of  extruded  or  rolled  bars  of  special  form  is 
often  adopted  in  the  manufacture  of  small  parts  for  adding 
machines,  typewriters,  counters,  and  other  similar  mechanisms. 

An  illustration  of  this  point  occurred  in  a  large  plant  which 
makes  small  duplicate  parts.  Several  of  these  parts  were  made 
of  brass  castings  because  of  the  lower  cost  of  machining,  but  the 
price  of  copper  began  to  rise  and  was  soon  about  double  its 
normal  price.  It  was  decided  to  substitute  cast  iron  for  brass 
because  the  difference  in  the  cost  of  machining  was  less  than  the 
difference  in  the  market  price  of  the  materials.  Luckily,  an 
investigation  was  made  before  the  change  went  into  effect. 
This  plant  had  its  own  brass  foundry  but  no  iron  foundry. 
It  was  discovered  that  the  foundry  had  purchased  no  copper 


34  INTERCHANGEABLE   MANUFACTURING 

for  several  years.  In  fact,  a  large  stock  of  pig  copper  had  been 
stored  in  a  shed  and  was  never  touched.  Another  department 
of  this  plant  was  engaged  in  making  copper  matrices  by  a  plat- 
ing process,  and  the  trimmings  from  these  supplied  all  the  pure 
copper  which  the  foundry  required.  This,  with  scrap  brass 
stock  from  other  departments,  made  it  unnecessary  to  purchase 
any  metal  for  the  brass  foundry  in  the  open  market.  Needless 
to  say,  no  change  was  made  in  the  material  of  the  castings. 
This  incident  indicates  in  some  degree  the  many  factors  that 
must  be  considered  to  secure  genuine  economy.  It  is  not  a 
matter  of  mere  addition  and  subtraction;  every  existing  con- 
dition must  be  taken  into  account. 

Weight  of  Finished  Product.  Whenever  the  weight  of  the 
finished  product  is  an  important  consideration,  as  with  auto- 
mobiles, etc.,  the  materials  used  in  making  it  must  be  of  a  better 
grade  than  when  the  weight  is  less  important.  In  every  case, 
the  materials  specified  must  be  sufficiently  strong  and  rigid 
to  hold  their  form  throughout  the  normal  life  of  the  mechanism. 
Thus,  the  detailed  design  of  the  various  components  is  governed 
to  a  great  extent  by  the  nature  of  the  materials  which  are  used 
in  their  manufacture.  For  example,  if  forged  steel  is  substituted 
for  cast  iron,  the  component  will  be  of  much  lighter  design. 

Service  Required.  The  composition  of  the  materials  used  is 
governed  by  the  nature  of  the  service  which  the  part  must  render. 
One  that  is  subjected  to  excessive  wear  must  be  made  of  material 
hard  or  tough  enough  to  withstand  it.  Material  for  parts  liable 
to  corrosion  or  other  chemical  action  must  be  of  the  proper 
composition  to  counteract  it.  Material  for  parts  under  constant 
vibration  must  not  crystallize  readily.  In  every  event,  the 
materials  must  be  selected  to  withstand  both  the  use  and  abuse 
which  they  will  eventually  meet. 

It  is  of  interest  to  note,  as  an  indication  of  the  importance 
of  materials  in  relation  to  the  total  cost  of  production,  that 
census  statistics  show  that  the  cost  of  materials  —  direct  and 
indirect  —  is  from  30  to  60  per  cent  of  the  selling  price  of  the 
majority  of  mechanical  products  which  are  manufactured  in 
this  country. 


MACHINE  DESIGN  35 

Clearances  and  Tolerances.  The  establishment  of  suitable 
clearances  and  tolerances  is  a  vital,  if  not  the  most  vital,  factor 
in  the  manufacturing  design.  Tolerances  are,  in  many  respects, 
like  laws.  There  are  two  classes  of  laws.  One  is  so  severe  and 
exacting  in  its  nature  that  it  cannot  be  enforced,  and  soon  falls 
into  disrepute  and  is  disregarded,  even  though  it  remains  on  the 
statute  books.  The  other  is  drawn  up  with  a  full  understanding 
of  existing  conditions,  and  its  justice  to  all  concerned  is  so 
evident  that  it  is  readily  and  consistently  enforced. 

Similarly,  tolerances  fall  into  two  classes.  Those  which 
represent  the  extreme  conditions  of  accuracy  obtainable  from 
the  equipment  under  ideal  conditions  can  be  specified  without 
regard  to  the  functional  requirements  of  the  product.  In  such 
cases  they,  too,  soon  fall  into  disrepute  and  are  disregarded, 
even  though  they  still  remain  on  the  drawings.  On  the  other 
hand,  tolerances  are  readily  and  consistently  maintained  when 
they  represent  the  widest  variations  that  the  functioning  of  the 
mechanism  will  safely  permit. 

Liberal  tolerances  and  clearances  result  in  easier  manu- 
facturing conditions  of  every  sort  and  thus  promote  economy; 
they  make  quantity  production  possible.  The  serviceable 
life  of  tools  depends  directly  on  the  extent  of  the  tolerances. 
Every  exacting  tolerance  is  a  direct  check  on  the  economical 
and  rapid  production  of  the  mechanism.  On  the  other  hand, 
if  the  functional  requirements  do  not  permit  wide  tolerances, 
the  functional  requirements  must  prevail. 

It  is  evident,  then,  that  the  construction  must  be  carefully 
studied  so  that  the  manufacturing  design  will  permit  the  widest 
possible  tolerances.  It  is  only  in  exceptional  cases  that  a  mechan- 
ism cannot  be  modified  so  as  to  retain  all  functional  advantages 
and  yet  allow  liberal  tolerances  on  the  majority  of  its  dimen- 
sions. Very  often,  when  there  is  a  severe  functional  requirement 
to  maintain,  the  introduction  of  simple  means  of  adjustment 
promotes  easier  manufacturing  conditions.  In  other  cases, 
a  system  of  selective  assembly  is  more  desirable. 

Applying  Interchangeable  Principle.  The  designer  must  de- 
termine which  parts  will  be  interchangeable.  Interchangeability 


36  INTERCHANGEABLE   MANUFACTURING 

can  be  carried  too  far  and  thus  allowed  to  defeat  its  own  purpose 
as  noted  in  a  previous  chapter.  Interchangeability  and  liberal 
maximum  clearances  are  closely  connected.  Whenever  reason- 
able clearances  are  out  of  the  question  on  certain  components, 
these  parts  are  not  suitable  ones  to  be  manufactured  on  an  inter- 
changeable basis.  In  this  matter,  the  relative  accuracy  of  the 
available  equipment  plays  a  large  part.  For  example,  if  the 
surfaces  are  elementary  and  can  be  finished  by  a  simple  grinding 
operation,  much  closer  tolerances  can  be  economically  main- 
tained than  if  they  are  composite  and  require  milling  or  turning 
operations.  The  variations  on  work  finished  by  grinding  are 
about  one-third  those  resulting  from  milling  and  one-half  those 
from  turning;  and  the  effort  expended  is  no  greater.  On  the 
other  hand,  grinding  is  not  always  suitable  nor  possible.  There- 
fore, in  determining  whether  or  not  certain  required  conditions 
permit  reasonable  tolerances,  the  designer  must  consider  pos- 
sible methods  of  manufacture  and  must  be  well  informed 
regarding  the  normal  variations  which  result  from  them  in 
actual  practice. 

This  knowledge  is  the  outcome  of  experience  in  checking  and 
analyzing  results  previously  secured.  This  is  a  matter  to  which 
little  attention  has  been  paid  in  the  past.  For  example,  in  a 
large  and  long-established  plant,  where  many  milling  operations 
are  performed,  it  had  been  assumed  that  these  operations  were 
maintained  within  a  tolerance  of  o.ooi  inch.  Actual  measure- 
ments brought  out  the  fact  that  the  normal  variation  was  over 
three  times  as  great  as  that,  and  always  had  been.  A  similar 
misconception  of  actual  conditions  was  apparent  in  the  majority 
of  shops  engaged  in  government  work  during  the  recent  war. 
When  their  product  was  actually  checked  by  limit  gages  and 
held  to  the  specified  tolerances,  a  variation  of  0.002  or  0.003  incQ 
was  found  to  be  an  extremely  small  manufacturing  tolerance. 
It  is,  therefore,  one  of  the  duties  of  the  maker  of  the  manu- 
facturing design  to  specify  the  parts  which  are  to  be  made  inter- 
changeable, those  to  be  selectively  assembled,  and  those  to  be 
fitted  to  each  other.  Careful  attention  to  this  detail  saves  much 
wasted  effort  in  the  shops  subsequently. 


MACHINE   DESIGN  37 

Advantages  of  Unit  Assembly  Construction.  Almost  every 
mechanism  can  be  subdivided  into  smaller  units  which  are  dis- 
tinct in  their  purpose.  For  example,  an  automobile  contains 
an  engine,  transmission,  axle  drive,  carburetor,  magneto,  etc., 
which  are  assembled  and  tested  as  units  and  later  assembled 
into  the  completed  car.  In  like  manner  a  typewriter  is  sub- 
divided into  the  carriage,  the  escapement,  the  type-bar  and  the 
segment  assembly,  etc.  The  assembly  is  greatly  facilitated  if 
the  design  of  the  mechanism  permits  such  unit  assembly  con- 
struction; and  efforts  should  be  made  to  obtain  this  result 
whenever  practicable. 

There  are  many  other  advantages  of  this  unit  assembly  con- 
struction. Not  only  the  various  manufacturing  departments 
of  one  factory  but  also  entire  plants  are  specializing  more  and 
more.  The  automobile  has  hastened  this  trend  more  than  any 
other  one  thing.  Where  such  unit  assemblies  are  of  equal  value 
on  several  articles,  separate  plants  spring  up  to  produce  them  as 
a  specialty.  This  gives  the  benefits  of  quantity  production 
where  otherwise  they  would  not  exist.  Therefore,  as  a  direct 
result  of  unit  assembly  construction,  there  are  separate  plants 
specializing  in  engines,  rear  axles,  carburetors,  magnetos,  etc., 
for  automobiles;  ball  and  roller  bearings  for  all  types  of  machin- 
ery; and  many  other  similar  specialized  products. 

Standardization  of  Parts.  Another  practice  which  allows  the 
benefits  of  quantity  production  to  be  obtained  in  the  production 
of  smaller  numbers  of  complete  mechanisms  is  the  standard- 
ization of  many  of  the  individual  components.  For  example, 
most  manufacturing  concerns  have  standardized  their  screws, 
nuts,  studs,  rivets,  and  others  mall  parts.  The  majority  of 
machine  tool  builders  also  standardize  their  handwheels,  mi- 
crometer thimbles,  gears,  tool-holders,  work-arbors,  etc.  A  good 
illustration  of  the  economy  of  this  practice  is  found  in  the  ex- 
perience of  one  plant  which  originally  manufactured  over  one 
hundred  and  fifty  special  screws  and  studs  for  its  particular 
product.  Little  effort  was  required  to  reduce  this  number  to 
less  than  half,  thus  increasing  the  rate  of  production  of  these 
parts  and  also  reducing  the  stock  of  spare  parts.  This  practice 


38  INTERCHANGEABLE   MANUFACTURING 

is  extending  to  larger  and  more  important  components.  Not 
only  are  similar  parts  produced  by  individual  plants  being 
standardized,  but  parts  used  in  common  by  several  manufactur- 
ers are  also  standardized  and  often  manufactured  as  specialties 
by  other  concerns. 

Designing  for  Assembling  and  Service.  The  design  must  per- 
mit the  ready  assembly  of  the  product.  Parts  which  require 
attention  in  service  must  be  accessible.  .Attention  to  these 
details  reduces  assembling  and  service  costs,  and  these  must  be 
considered  to  insure  ultimate  economy. 

The  service  requirements  are  the  most  difficult  to  determine. 
Time  alone  brings  the  desired  information.  Experiments  and 
endurance  tests  in  the  factory  are  insufficient  to  give  it.  After  a 
mechanism  is  on  the  market,  it  receives  use  and  abuse  that  the 
makers  never  dreamed  of.  Yet  if  the  product  fails  under  these 
unforeseen  conditions,  the  manufacturing  plant  is  blamed. 
Naturally  the  nature  of  the  commodity  determines  what  sort 
of  service  it  must  render.  The  service  requirements  of  an 
automobile  are  distinct  from  those  of  a  typewriter;  those  of  a 
precision  machine  tool  —  which  is  supposedly  used  by  skilled 
mechanics  only  —  differ  from  those  of  a  lawn-mower;  etc. 

The  service  requirements  include  the  protection  of  the  working 
parts  from  dirt  and  other  foreign  matter,  the  provision  of  proper 
lubricating  facilities,  and  the  protection  of  the  operator  from 
moving  parts.  The  question  of  the  best  preservative  finishes, 
such  as  japanning,  plating,  painting,  etc.,  must  also  be  answered 
to  meet  the  service  requirements,  both  of  use  and  appearance. 
For  these  and  many  other  similar  problems  a  solution  is  sought 
that  will  result  in  the  maximum  amount  of  service  at  a  minimum 
expense. 

It  should  be  clearly  understood  that  the  manufacturing  design 
is  not  undertaken  with  the  idea  of  wilfully  altering  the  functional 
design,  but  is*made  to  facilitate  manufacture  and  to  furnish  as 
much  as  possible  of  that  vast  amount  of  detailed  information 
previously  brought  to  the  productive  work  by  the  mechanic 
who  carried  out  the  inventor's  ideas.  The  alterations  made  in 
the  functional  design  by  the  manufacturing  design  should  not 


MACHINE   DESIGN  39 

be  looked  on  as  any  criticism  of  the  original  lay-out.  Each  has 
its  distinct  purpose  to  perform.  Many  large  plants  recognize 
clearly  the  difference  between  the  two  types  of  designing  and 
maintain  separate  departments  for  each.  The  original  research 
and  inventive  work  is  carried  on  independently  of  the  factory 
operations.  New  or  improved  designs  are  turned  over  to  the 
factory  organization  where  they  are  redesigned  to  meet  the 
manufacturing  and  service  needs. 


CHAPTER  IV 
PURPOSE    OF    MODELS 

A  MODEL  mechanism,  constructed  personally  by  the  inventor, 
or  by  the  workmen  under  his  immediate  direction,  was  the  original 
form  of  making  and  recording  a  new  design.  The  introduction 
and  development  of  mechanical  drawings  superseded  many  of 
the  functions  previously  performed  by  the  model.  At  the 
present  time,  therefore,  the  practice  of  developing  models  has 
been  relegated  to  a  comparatively  insignificant  place  in  most 
lines  of  manufacturing.  They  are  still  employed  to  a  limited 
degree,  however,  by  several  manufacturers  for  a  variety  of 
purposes. 

The  primary  purpose  of  any  model  at  the  present  time  is 
to  prove  —  not  to  originate  —  a  new  or  improved  design  that 
has  been  developed  only  on  paper.  It  may  be  either  to  prove 
the  possibility  of  the  functional  design  or  to  check  the  manu- 
facturing design.  This  may  be  done  by  a  single  mechanism 
in  some  cases,  or  several  duplicate  mechanisms  may  be  required 
to  prove  its  operation  under  various  service  conditions. 

Manufacturing  models  may  be  used  for  one  or  more  of  the 
following  purposes:  First,  to  check  the  operation  of  the  manu- 
facturing design  against  the  experimental  model;  second,  to 
prove  the  manufacturing  design  in  regard  to  the  service  require- 
ments; third,  to  test  the*  manufacturing  tolerances  which  may 
be  contemplated;  and  fourth,  to  create  a  physical  standard  of 
precision  for  future  manufacturing. 

Manufacturing  Model  to  Test  Functioning.  A  manufacturing 
model  used  to  test  the  functioning  of  the  manufacturing  design 
is  merely  a  sample  mechanism  constructed  in  the  tool-room  or 
machine  shop  to  detect  as  many  faults  as  possible  in  the  design 
or  to  discover  possible  errors  in  the  component  drawings.  It  is 
essentially  a  precautionary  measure.  It  is  more  economical  to 

40 


PURPOSE   OF   MODELS  41 

detect  and  correct  a  fault  on  one  sample  than  it  is  to  salvage  a 
large  number  of  parts  after  production  is  started,  with  the  addi- 
tional expense  of  correcting  the  manufacturing  equipment. 
After  such  a  model  has  demonstrated  the  success  of  the  manu- 
facturing design  and  the  correctness  of  the  component  drawings, 
its  purpose  has  been  achieved.  Its  future  disposition  is  a  matter 
of  little  moment. 

Such  a  model  is  seldom  necessary  on  simple  mechanisms  that 
are  merely  new  combinations  of  old  and  proved  mechanical 
movements,  or  on  minor  variations  of  proved  designs,  such  as 
standard  motors,  dynamos,  various  types  of  engines,  and  many 
machine  tools.  The  actions  of  such  mechanisms  under  many 
conditions  have  been  so  well  established  that  practically  all  of 
the  necessary  experimental  work  can  be  accomplished  on  paper. 
On  many  other  mechanisms,  however,  such  as  typewriters,  add- 
ing machines,  small  arms,  watches,  etc.,  the  mechanical  move- 
ments of  which  are  delicate  and  intricate  and  not  so  positive, 
manufacturing  models  are  a  vital  necessity.  In  general,  such  a 
model  will  be  constructed  when  the  insurance  against  the  possi- 
ble errors  in  the  design  is  worth  the  expense  entailed.  For  this 
reason  it  is  often  customary  to  build  a  pilot  machine  before 
putting  through  a  lot  of  new  or  special  machines. 

If  a  new  commodity  is  designed,  particularly  if  it  is  to  fill  a 
new  demand,  it  is  advisable  to  determine  its  action  in  actual 
service  before  extensive  productive  operations  are  far  advanced. 
The  only  sure  method  of  obtaining  this  information  is  to  have 
one  or  more  mechanisms  built  and  operated  under  the  condi- 
tions with  which  they  are  expected  to  contend.  The  manu- 
facturing model,  which  is  built  to  test  the  manufacturing  design, 
is  often  used  for  this  purpose.  As  a  matter  of  fact,  the  test  for 
functioning  should  also  include  the  tests  for  service  require- 
ments, inasmuch  as  this  factor  of  functioning  should  include  the 
measure  of  service  which  the  mechanism  must  render  throughout 
its  normal  life. 

For  example,  a  large  plant  in  the  Middle  West  goes  thor- 
oughly into  this  preliminary  work  on  all  new  models.  Three 
successive  designs  are  developed  and  tested.  First,  the  func- 


42  INTERCHANGEABLE   MANUFACTURING 

tional  or  inventive  design  is  made  and  tested.  Second,  the 
manufacturing  design  is  carefully  developed  and  tested  by 
means  of  a  manufacturing  model.  When  this  last  design  seems 
satisfactory,  it  is  turned  over  to  the  tool  designing  department 
which  goes  over  it  a  third  time  solely  to  simplify  the  tooling  and 
mechanical  productive  operations.  The  changes  made  at  this 
time,  however,  affect  minor  details  only.  From  twenty-five  to 
fifty  mechanisms  are  built  to  this  design  and  sent  into  the  field 
for  actual  service.  These  last  models  must  give  satisfactory 
service  for  from  one  to  two  years  before  further  preparations 
for  manufacture  are  considered.  It  is  of  interest  to  note  that 
the  manufacturing  equipment  provided  by  this  factory  is  com- 
plete; also  that  changes  here  in  the  process  of  manufacture  or 
in  the  product  under  production  are  very  rare. 

Many  of  the  so-called  improvements  in  new  commodities 
which  result  in  frequent  modifications  of  the  product  under 
manufacture  are  only  steps  taken  to  correct  mistakes,  omis- 
sions, and  other  faults  due  in  large  measure  to  neglect  of  the 
manufacturing  design  (both  neglect  to  make  and  neglect  to  test 
it)  because  of  haste  to  rush  into  actual  production.  This  has 
been  forcibly  brought  out  by  the  conditions  which  developed  in 
the  manufacture  of  many  devices  during  the  war. 

Models  to  Test  Tolerances.  It  is  desirable  to  know  at  the 
earliest  possible  moment  whether  or  not  the  specified  tolerances 
define  the  limits  of  parts  that  will  function  properly.  The 
sooner  this  information  is  obtained,  the  sooner  can  efforts  be 
concentrated  on  problems  of  production  alone.  Until  this 
matter  is  settled  within  a  reasonable  degree  of  certainty,  each 
problem  in  production  is  complicated  by  many  considerations 
relating  to  the  design  and  tolerances.  This  causes  innumer- 
able revisions  on  the  component  drawings  with  the  attendant 
changes  in  tools,  fixtures,  and  gages,  resulting  in  delays  in 
production  and  additional  expense. 

Some  concerns  try  to  solve  this  problem  by  carefully  build- 
ing several  models  which  represent  as  closely  as  practicable  the 
extreme  conditions  permitted  by  the  component  drawings. 
These  model  parts  are  assembled  and  reassembled,  and  tested 


PURPOSE   OF   MODELS  43 

for  operation  after  each  assembly.  Necessary  alterations  of 
the  drawings  are  made  before  manufacturing  operations  are 
under  way.  This  practice  is,  naturally,  expensive,  as  each  of 
these  models  costs  much  more  to  construct  than  any  of  the 
preceding  ones.  However,  if  insurance  against  future  changes 
is  worth  the  expense,  the  practice  is  well  worth  while. 

One  typewriter  manufacturer  makes  a  practice  of  building 
from  six  to  ten  sets  of  model  parts  before  any  new  or  revised 
machine  is  manufactured.  When  only  one  unit  assembly  is 
affected,  model  parts  for  that  mechanism  only  are  made.  These 
parts  are  then  sent  to  the  assembling  department  for  trial. 
Except  on  purely  experimental  models,  the  men  who  make  the 
parts  are  not  allowed  to  assemble  them.  No  effort  is  made  to 
do  anything  more  than  to  duplicate  the  kind  of  work  normally 
produced  in  the  manufacturing  departments.  The  parts  are 
not  cornered  or  burred  unless  that  operation  is  required  in 
manufacture.  In  other  words,  the  attempt  is  made  to  determine 
how  little  effort  will  be  required  to  manufacture  the  mechanism 
satisfactorily.  The  sizes  of  these  parts  usually  cover  the  entire 
range  between  the  specified  limits,  but  no  distinct  effort  is  made 
to  have  them  meet  either  limit  exactly.  Any  combination  of 
these  parts  must  assemble  and  operate  properly  before  changes 
or  new  models  are  adopted.  This  practice  has  saved  the  com- 
pany several  times  from  making  unnecessary  and  improper 
changes. 

Model  for  Standard  of  Precision.  The  component  drawings 
give  many  dimensions.  Strictly  speaking,  the  expressed  di- 
mensions represent  absolute  sizes.  Dimensions  of  elementary 
surfaces  can  be  produced  and  reproduced  within  relatively  small 
limits  of  error.  Often,  however,  these  dimensions  define  de- 
veloped and  complicated  profiles,  locations,  and  other  com- 
posite surfaces  which  cannot  be  reproduced  as  readily.  Yet 
they  must  be  reproduced  many  times  over  in  the  course  of  manu- 
facture to  within  relatively  small  limits  of  error. 

A  choice  must  be  made  between  accuracy  and  precision  at 
the  outset.  In  the  case  of  elementary  surfaces,  accuracy  is 
usually  the  better  choice;  but  for  many  composite  surfaces, 


44  INTERCHANGEABLE   MANUFACTURING 

precision  will  often  give  the  quicker,  more  economical,  and 
more  practical  results.  On  the  other  hand,  it  must  be  clearly 
understood  that  when  precision  is  chosen,  it  becomes  prac- 
tically impossible  for  another  plant,  working  in  entire  indepen- 
dence of  the  first,  to  produce  a  common  interchangeable  product. 
If  more  than  one  plant  is  engaged  on  the  production,  they  must 
maintain  close  relations  in  almost  every  detail  of  the  work.  In 
order  to  maintain  this  precision  within  reasonably  close  limits, 
physical  standards  of  some  sort  must  be  provided  at  the  very 
beginning.  By  developing  a  model  for  this  purpose,  two  results 
can  be  accomplished  at  the  same  time.  Such  a  model  will  test 
the  manufacturing  design  for  functioning,  and  will  also  provide 
the  desired  physical  standards. 

This  model  must  be  made  with  the  greatest  care  and  should 
represent  the  "danger  zone"  —  that  is,  in  most  cases,  the 
maximum  metal  or  minimum  clearance  conditions.  It  is  evi- 
dent that  these  sizes  are  the  most  important.  They  control 
the  interchangeability.  No  cuts,  other  than  slight  cornering 
and  similar  burring  operations,  should  be  made  with  a  hand 
tool,  such  as  a  file.  Whenever  the  contour  of  the  surface  in 
question  is  important,  templets  should  be  made  to  check  the 
special  tools  used.  These  templets  are  an  integral  part  of  the 
model.  All  important  locations  of  holes  should  be  established 
from  master  plates.  Templets,  master  plates,  etc.,  as  well  as 
the  model  parts,  are  invaluable  when  the  equipment  is  built; 
properly  utilized,  they  will  insure  a  high  degree  of  uniformity 
at  relatively  small  expense.  Such  a  model  must  be  used  with 
the  greatest  care  and  becomes  the  court  of  last  appeal  in  many 
of  the  perplexing  questions  which  inevitably  arise  in  the  manu- 
facturing departments  of  a  plant  engaged  in  producing  an  inter- 
changeable product  in  large  quantities. 

This  practice  in  regard  to  models  is  extensively  followed  in 
the  manufacture  of  small  arms.  As  stated  before,  it  has  a 
certain  disadvantage  when  more  than  one  factory  is  involved. 
As  it  is  impossible  to  duplicate  the  model  exactly,  one  of  two 
courses  is  open.  Either  one  model  is  standard  for  all  plants  — 
which  entails  much  lost  time  in  referring  many  detailed  ques- 


PURPOSE   OF   MODELS  45 

tions  back  to  the  central  plant  —  or  additional  models  may 
be  made,  which  results  in  different  basic  standards  at  the  vari- 
ous plants.  If  the  second  course  is  followed,  and  all  parts  of  all 
models  are  mutually  interchangeable,  the  product  of  the  various 
factories  will  be  interchangeable.  However,  this  results  in 
reducing  the  amount  of  the  absolute  tolerances  available  for 
manufacturing  variations,  as  the  variations  in  the  different 
models  consume  a  certain  amount.  In  cases  where  the  func- 
tional conditions  are  exacting,  this  method  is  often  found  im- 
practicable. On  the  other  hand,  if  the  design  of  the  mechanism 
is  such  that  the  absolute  tolerances  are  liberal,  the  second 
method  gives  an  economical  solution. 

All  the  foregoing  model  work,  regardless  of  its  purpose,  is 
essentially  a  preliminary  measure  in  the  manufacture  of  a  new 
or  revised  product.  Properly  conducted,  it  will  stabilize  the 
manufacturing  design  at  a  minimum  expense.  It  takes  con- 
siderable time,  however,  and  that  is  one  reason  why  models 
are  not  more  extensively  employed.  For  any  commodity  that 
is  already  under  manufacture  and  the  design  of  which  is  already 
standardized,  models  are  of  doubtful  value. 


CHAPTER  V 
PRINCIPLES    IN    MAKING    COMPONENT    DRAWINGS 

THE  art  of  expressing  mechanical  information  by  means  of 
drawings  is  still  in  the  process  of  evolution.  Many  details  have 
become  conventionalized,  yet  these  comprise  little  more  than 
the  alphabet  of  the  language  of  drawings  and  relate  principally 
to  conventional  meanings  of  the  lines,  figures,  and  relative 
locations  of  the  several  projections  which  go  to  complete  the 
drawings.  Such,  for  example,  are  the  full  lines  which  represent 
the  visible  outlines  of  the  part;  the  dotted  lines  which  represent 
the  hidden  outlines;  the  light  dot-and-dash  lines  which  indicate 
center  lines;  the  light  dimension  lines  —  and  all  the  other  con- 
ventional lines  and  characters  which  are  employed.  The  third- 
angle  projection  is  also  fairly  well  established  in  mechanical 
drawing.  This  branch  of  drawing  is  fully  covered  in  text-books, 
so  no  further  mention  of  it  will  be  made  here.  The  subject  of 
dimensioning,  however,  is  so  incompletely  covered  that  this 
chapter  will  be  devoted  to  a  detailed  discussion  of  this  subject. 
The  addition  of  tolerances  on  component  drawings  has  created 
new  problems  which  have  not,  as  yet,  been  fully  solved,  and 
which,  therefore,  require  considerable  and  thoughtful  study. 

The  matter  of  dimensioning,  as  given  in  books  and  taught 
in  various  schools,  receives  only  minor  attention.  Little  more 
than  the  a  b  c  of  the  subject  is  taught.  In  actual  practice  — 
particularly  where  tolerances  are  involved  —  so  many  different 
conditions  are  to  be  met,  so  many  different  shades  of  meaning 
must  be  clearly  expressed,  and  so  many  different  types  of  work- 
men must  be  informed  by  these  drawings  that  this  alphabet 
must  be  fully  understood  and  carefully  used  to  enable  it  to  serve 
its  purpose.  It  is  necessary,  in  order  to  consider  intelligently 
this  subject  of  dimensioning  with  tolerances,  to  discard  all  school 
training  in  the  application  of  dimension  lines,  etc. 

46 


COMPONENT  DRAWINGS  47 

The  main  purpose  of  a  mechanical  drawing  is  to  express  or 
record  information.  This  information  is  of  many  kinds  and  is 
used  for  many  purposes.  The  drawing,  to  be  correct,  must 
clearly  and  consistently  express  the  particular  information  re- 
quired to  serve  its  specific  purpose.  For  example,  a  type  of 
drawing  that  may  be  correct  for  the  use  of  a  toolmaker  in  build- 
ing a  jig  may  be  incorrect  for  the  use  of  a  machine  operator  in 
the  manufacturing  department  engaged  in  quantity  production. 
Inasmuch  as  the  drawings  are  the  written  or  pictured  expres- 
sion of  the  design,  they  may  be  roughly  classified  into  func- 
tional drawings  and  manufacturing  or  component  drawings. 

Functional  Drawings.  The  functional  drawing,  like  the  func- 
tional design,  primarily  expresses  the  functional  conditions  to 
be  maintained.  The  detailed  information  relating  to  many  of 
the  manufacturing  problems  that  are  involved  which  does  not 
appear  on  these  drawings  is  supplied  by  the  mechanic  who  uses 
them.  Thus,  in  those  cases  where  only  a  few  special  mechan- 
isms, or  jigs,  fixtures,  tools,  etc.,  are  to  be  made  in  a  general 
machine  shop  or  tool-room,  where  the  type  of  workman  is  such 
that  this  detailed  information  is  unnecessary,  functional  draw- 
ings only  are  required.  Such  drawings  need  not  express  toler- 
ances, clearances,  and  other  minor  details  so  essential  on  the 
manufacturing  drawings.  For  example,  a  notation  such  as 
"drive  fit"  or  " sliding  fit"  is  sufficient  to  indicate  and  obtain 
the  desired  results.  Yet,  even  here,  if  the  drawings  are  to  serve 
their  purpose  efficiently,  the  information  given  must  be  so 
expressed  that  it  may  be  used  directly.  In  order  to  attain  this 
end,  every  line  drawn  and  every  dimension  expressed  must  be 
made  with  a  full  understanding  of  the  final  results  required  and 
of  the  means  to  be  employed  to  obtain  them. 

Manufacturing  Drawings.  The  manufacturing  drawings,  to 
be  complete,  must  express  all  suitable  information  that  is  avail- 
able. For  the  purposes  of  the  present  discussion,  we  will  con- 
fine ourselves  to  component  drawings  of  an  interchangeable 
product.  As  stated  in  a  preceding  chapter,  the  proper  dimen- 
sioning of  component  draiwngs  with  tolerances  is  a  mathe- 
matical problem.  Five  laws  are  given,  which,  if  carefully 


48  INTERCHANGEABLE  MANUFACTURING 

observed,  will  simplify  many  of  the  equipment  and  production 
problems. 

Laws  of  Dimensioning,  i.  In  interchangeable  manufactur- 
ing there  is  only  one  dimension  (or  group  of  dimensions)  in  the 
same  straight  line  which  can  be  controlled  within  fixed  toler- 
ances. This  is  the  distance  between  the  cutting  surface  of  the 
tool  and  the  locating  or  registering  surface  of  the  part  being 
machined.  Therefore,  it  is  incorrect  to  locate  any  point  or 
surface  with  tolerances  from  more  than  one  point  in  the  same 
straight  line. 

2.  Dimensions  should  be  given  between  those  points  which 
it  is  essential  to  hold  in  a  specific  relation  to  each  other.    The 
majority  of  dimensions,  however,   are  relatively  unimportant 
in  this  respect.    It  is  good  practice  to  establish  common  location 
points  in  each  plane  and  to  give,  as  far  as  possible,  all  such 
dimensions  from  these  points. 

3.  The  basic  dimensions  given  on  component  drawings  for 
interchangeable  parts  should  be,  except  for  force  fits  and  other 
unusual  conditions,  the  maximum  metal  sizes.    The  direct  com- 
parison of  the  basic  sizes  should  check  the  danger  zone,  which 
is  the  minimum  clearance  condition  in  the  majority  of  cases. 
It  is  evident  that  these  sizes  are  the  most  important  ones,  as 
they  control  the  interchangeability,  and  they  should  be  the  first 
determined.    Once  established,  they  should  remain  fixed  if  the 
mechanism  functions  properly  and  the  design  is  unchanged. 
The  direction  of  the  tolerances,  then,  would  be  such  as  to  recede 
from  the  danger  zone.    In  the  majority  of  cases,  this  means  that 
the  direction  of  the  tolerances  is  such  as  will  increase  the  clear- 
ance.   For  force  fits,  such  as  taper  keys,  etc.,  the  basic  dimen- 
sions determine  the  minimum  interference,  while  the  tolerances 
limit  the  maximum  interference. 

4.  Dimensions  must  not  be  duplicated  between  the  same 
points.     The  duplication  of  dimensions  causes  much  needless 
trouble,  due  to  changes  being  made  in  one  place  and  not  in  the 
others.     It  causes  less  trouble  to  search  a  drawing  to  find  a 
dimension  than  it  does  to  have  them  duplicated  and  more 
readily  found  but  inconsistent. 


COMPONENT   DRAWINGS 


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COMPONENT   DRAWINGS 


tions,  it  is  evident  from  the  foregoing  that  a  different  product 
would  be  received  from  each  plant.  The  example  given  is  the 
simplest  one  possible.  As  the  parts  become  more  complex,  and 
the  number  of  dimensions  increase,  the  number  of  different  com- 
binations possible  and  the  extent  of  the  variations  in  size  that 
will  develop  also  increase. 

Fig.  4  shows  the  correct  way  to  dimension  this  part  if  the 
length  of  the  body  and  the  length  of  the  stem  are  the  essential 
dimensions.  Fig.  -5  is  the  correct  way  if  the  length  of  the  body 
and  the  length  over  all 
are  the  most  important. 
Fig.  6  is  correct  if  the 
length  of  the  stem  and 
the  length  over  all  are 
the  most  important. 

If  the  part  is  dimen- 
sioned in  accordance  with 
either  Fig.  4,  Fig.  5,  or 
Fig.  6,  the  product  from 
any  number  of  factories 
should  be  alike.  There  is 
now  no  excuse  for  them  to 
misinterpret  the  meaning  of  the  drawing.  The  point  may  be 
raised  that  the  manufacturer  should  study  the  drawing  to  deter- 
mine what  his  sequence  of  operations  should  be  in  order  to  main- 
tain all  dimensions  and  tolerances  given.  On  such  a  simple 
part  as  was  given  for  the  first  example,  this  would  not  be  difficult. 
On  a  more  complicated  piece,  however,  it  would  be  almost  im- 
possible. Such  conditions  occur  when  the  draftsman  makes  little 
or  no  effort  to  reduce  as  many  surfaces  as  possible  to  elementary 
ones.  Furthermore,  when  the  manufacturer  or  workman  sees 
such  dimensions  on  a  component  drawing,  he  is  justified  in  as- 
suming that  the  designer  or  draftsman  who  made  them  had  little 
or  no  idea  as  to  the  essential  conditions  to  be  maintained.  In 
such  cases,  the  sequence  of  operations  and  the  register  points  for 
machining  will  be  established  to  facilitate  production,  or  to 
suit  the  ideas  of  individuals  as  to  the  most  essential  conditions. 


Machinery 


Fig.  7.     A  Third  Interpretation  of  Dimen- 
sioning in  Fig.  1 


INTERCHANGEABLE   MANUFACTURING 


Often,  this  will  result  in  some  of  the  operations  on  a  component 
being  arranged  to  suit  one  idea,  while  the  remainder  are  com- 
pleted in  accordance  with  an  almost  diametrically  opposed  con- 
ception. It  cannot  be  too  strongly  impressed  upon  the  drafts- 
man that  when  a  drawing  leaves  his  hands  it  must  not  be  open 
to  more  than  one  interpretation.  This,  in  turn,  demands  that 
a  uniform  method  of  interpretation  be  adopted  and  published 
by  each  plant  for  the  guidance  of  all  concerned.  It  is  self-evident 
that  a  universal  method  of  interpretation  of  drawings  with 
tolerances  would  be  of  great  benefit  to  all  manufacturing  plants. 


ON  EACH  SIDE 


WIN.   CLEARANCE  0.005 
MAX.  CLEARANCE  0.  025*' 


A  \ 

FUNCTIONAL    BEARING  OR  FUNCTIONAL 

SURFACE 


(MIN.   CLEARANCE  0.002" 
IMAX.CLEARANCE  0.0121' 


Machinery 


Fig.  8.     Sketch  showing  Functional  Requirements  of  Slide 

This  is  a  field  where  the  various  engineering  societies,  working 
in  close  cooperation,  could  render  valuable  service. 

Violation  of  the  Second  Law.  Let  us  take  as  the  second 
example  the  slide  shown  in  the  sub-assembly,  Fig.  8.  This 
sketch  gives  the  functional  conditions  which  must  be  main- 
tained. It  is  well  to  note  that  it  is  a  very  desirable  practice  to 
add  to  a  set  of  component  drawings  a  series  of  sub-assemblies 
of  this  kind.  These  would  show  graphically  the  functional 
requirements  of  the  most  important  operating  members  of  the 
mechanism,  when  the  detail  drawings  are  insufficient,  in  them- 
selves, to  express  them  clearly.  Such  a  practice  will  prove  of 
great  assistance  in  limiting  the  interpretation  of  the  component 
drawings. 

Fig.  9  illustrates  a  common  method  of  dimensioning  such 
details.  This  is  wrong,  as  it  violates  the  second  law  previously 
stated.  These  parts  are  dimensioned  in  Fig.  10  in  accordance 


COMPONENT   DRAWINGS 


53 


with  the  foregoing  laws.  It  will  be  noted  that  all  dimensions 
for  height  are  given  from  the  bearing  surface  A,  which  is  the 
most  important  in  this  case.  If  the  slide  should  be  designed 
to  bear  at  B  instead  of  at  A ,  surface  B  would  become  the  most 
important,  and  the  various  dimensions  of  height  would  be  given 
from  there  instead  of  from  A.  The  same  functional  conditions 
(see  Fig.  8)  are  maintained  in  Figs.  9  and  10.  Attention  is 


Marhincry 


Fig.  9.     Incorrect  Dimensioning  of  Slide  shown  in  Fig.  8 


Fig.  10.     Proper  Dimensioning  of  Slide  shown  in  Fig.  8 

called  to  the  fact  that  in  Fig.  10  it  is  possible  to  allow  a  toler- 
ance of  o.o  10  inch  on  the  dimension  to  the  top  surface  B,  whereas 
in  Fig.  9  only  0.005  mch  can  be  allowed  as  a  manufacturing 
tolerance  when  making  this  cut. 

Increasing  Possibility  of  Draftsman's  Errors.  Thus,  the  im- 
proper and  careless  dimensioning  of  component  drawings  results 
directly  in  reducing  the  manufacturing  tolerances,  in  addition 
to  creating  uncertainty  by  not  indicating  the  essential  surfaces. 
Furthermore,  the  possibility  of  draftsman's  errors  is  greatly 


54 


INTERCHANGEABLE   MANUFACTURING 


increased  by  dimensioning  as  shown  in  Fig.  9  because  the  drafts- 
man, in  this  case,  niust  make  several  additions  and  subtractions 
of  basic  figures  and  tolerances  in  order  to  check  the  maximum 
and  minimum  clearances.  In  Fig.  10,  on  the  other  hand,  the 
direct  comparison  of  the  basic  dimensions  checks  the  minimum 
clearance.  The  maximum  clearance  is  readily  checked  by  adding 
the  sum  of  the  tolerances  to  this  minimum  clearance.  In  general, 
the  direct  comparison  of  the  basic  dimensions  should  establish 
the  minimum  clearances  between  elementary  surfaces  on  com- 
panion parts. 

No  mention  has  been  made  of  the  dimensions  of  width  in 
the  previous  example.     Strictly  speaking,  dimensions  so  given 


Machinery 


Fig.  11.    Graphical  Illustration  of  Application  of  Tolerance 

are  central  with  the  center  line.  Half  of  the  tolerances  for 
width  may  be  utilized  on  either  side  of  the  center  line.  This 
does  not  mean  that  the  surfaces  must  be  absolutely  central; 
one  side  can  be  made  to  the  maximum  dimension  and  the  other 
side  to  the  minimum.  In  general,  the  tolerances  should  be 
understood  to  establish  a  parallel  zone  of  acceptable  work,  all 
parts  falling  within  this  zone  being  acceptable.  Fig.  n  illus- 
trates how  the  dimensions  and  tolerances  in  Fig.  10  establish 
such  a  zone.  The  full  lines  show  the  basic  or  maximum  metal 
conditions,  while  the  dotted  lines  show  the  minimum  metal 
conditions. 

Inspection  Gage  Requirements.  In  the  previous  example, 
each  surface  has  been  considered  as  an  independent  elementary 
surface,  and  the  meaning  of  the  drawing  interpreted  accordingly. 
But  there  is  also  a  certain  condition  of  alignment  which  these 


COMPONENT   DRAWINGS  55 

various  surfaces  must  maintain  in  relation  to  each  other.  When 
considering  this  phase  of  the  subject,  the  surfaces  become  com- 
posite. Whenever  composite  surfaces  are  involved,  the  func- 
tional requirements  of  these  surfaces  must  be  taken  into  con- 
sideration. The  only  satisfactory  method  of  solving  such 
problems  is  in  terms  of  the  inspection  gage  requirements.  If 
the  succeeding  solutions  are  accepted,  the  accompanying  inter- 
pretations, expressed  in  terms  of  functional  gages,  must  also  be 
accepted. 

To  a  certain  extent,  the  amount  of  tolerance  required  to 
machine  a  given  surface  depends  on  the  methods  employed  to 
check  the  results  obtained.  For  example,  the  maximum  thick- 
ness of  the  tongue  of  the  slide  shown  in  Fig.  10  is  0.623  inch. 
If  this  thickness  is  checked  with  an  ordinary  snap  gage,  prac- 
tically the  entire  tolerance  is  available  for  variations  in  thick- 
ness. If,  however,  the  width  of  this  snap  gage  were  equal  to 
or  greater  than  the  length  of  the  tongue,  any  deviations  in  the 
surfaces  checked  from  true  parallel  planes  would  tend  to  pre- 
vent the  part  from  entering  the  gage.  In  this  case,  part  of  the 
tolerance  would  be  consumed  by  the  errors  in  alignment  of  the 
two  surfaces,  leaving  the  remainder  for  variations  in  the  dis- 
tance between  them. 

One  of  the  principal  reasons  for  providing  clearances  in  the 
design  is  to  discount  this  condition  of  misalignment.  In  develop- 
ing functional  gages  to  check  these  conditions,  therefore,  we  are 
justified  in  utilizing  a  fair  percentage  of  the  minimum  clearance. 
In  order  to  insure  strict  interchangeability,  the  functional  gage 
for  the  male  component  should  never  be  larger  than  the  func- 
tional gage  for  its  companion  female  component.  In  general, 
if  the  functional  gages  never  invade  this  minimum  clearance 
more  than  fifty  per  cent,  we  shall  remain  on  the  safe  side.  Con- 
ditions sometimes  arise,  of  course,  where  it  is  desirable  to  utilize 
a  greater  percentage  on  one  component  and  a  correspondingly 
lesser  percentage  on  the  other.  For  the  purposes  of  this  dis- 
cussion, however,  we  shall  assume  that  the  conditions  are  such 
that  a  maximum  of  fifty  per  cent  for  each  component  represents 
a  fair  distribution. 


50  INTERCHANGEABLE   MANUFACTURING 

The  dimensions  for  functional  gages  to  check  the  parts  shown 
in  Fig.  10  are  given  in  Fig.  12.  The  various  dimensions  of  the 
parts  should  first  be  checked  as  elementary  surfaces  with  limit 
gages  representing  the  specified  limits.  This  functional  gage 
would  then  be  employed  to  test  the  relative  alignment  of  these 
surfaces  necessary  to  insure  interchangeability. 


|<               2.998"             J 

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-\ 

Machinery 

Fig.  12.    Dimensions  for  Functional  Gages  for  Part  shown  in 
Fig.  10 


BEARING  OR  FUNCTIONAL 


TO  SHARP  CORNERS 


7 

Jl        (  MIN.  |CLEARANCE^0.002"^ON  EACH 

P"  t  MAX. 


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ff SURFACE 


HNH 


N.   CLEARANCE  O.OIo'J 
AX.  CLEARANCE  O.OSo" 


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MAX.  CLEARANCE  0.045  J    SIDE 


!!     (MIN.   CLEARANCE  0.025"  » 
~nP>AX.  CLEARANCE  0. 045  f 


ON 
EACH 
SIDE 


Machinery 


Fig.  13.    Sketch  showing  Functional  Requirements  of  Dovetail 
Slide 

Dimensioning  Composite  Surfaces.  Thus  far  we  have  been 
considering  parts  whose  surfaces  are  susceptible  of  individual 
checking  as  elementary  surfaces.  We  must  also  consider  parts 
whose  surfaces  cannot  be  resolved  into  elementary  ones  and 
checked  as  such.  Take,  for  example,  a  dovetail  slide,  such  as 
shown  in  Fig.  13,  which  introduces  an  angular  surface.  Such 
angular  surfaces  are  almost  always  composite  ones.  Great  care 
must  be  exercised  in  such  cases  to  avoid  compound  tolerances. 


COMPONENT   DRAWINGS 


57 


A  compound  tolerance  exists  when  the  application  of  a  toler- 
ance on  one  dimension  develops  a  variation  in  another  dimen- 
sion which  also  has  a  tolerance  specified.  Such  a  condition 
immediately  raises  the  question  as  to  whether  the  resultant 
variation  of  both  tolerances  is  permissible  or  whether  the  toler- 
ances specified  are  final  and  complete  for  their  respective  di- 
mensions. In  either  event,  confusion  and  misunderstanding  will 
result.  Here,  as  with  the  introduction  of  more  than  one  dimen- 
sion in  the  same  straight  line  (see  first  law  of  dimensioning)  to 
locate  a  given  surface,  the  final  results  will  depend  on  the  se- 
quence of  operations  adopted,  with  all  the  attendant  differences. 


Machinery 


Fig.  14.     Correct  Dimensioning  of  Dovetail  Slide  shown  in  Fig.  13 

As  stated  in  the  chapter  "  Principles  of  Interchangeable  Manu- 
facturing," in  making  component  drawings,  the  effort  should 
be  made  to  so  give  the  dimensions  and  necessary  tolerances  that 
it  would  be  possible  to  lay  out  one  —  and  only  one  — •  repre- 
sentation of  the  maximum  metal  condition  and  one  —  and  only 
one  —  minimum  metal  condition.  If  such  lay-outs  were  super- 
imposed, the  difference  between  them  would  represent  the  per- 
missible variation  on  every  surface.  If  a  few  such  lay-outs  are 
made,  it  will  soon  be  evident  that  there  are  always  a  number  of 
dimensions  that  should  be  given  without  tolerances. 

A  compound  tolerance  is  an  error  —  often  a  serious  one.  It 
can  and  should  always  be  eliminated.  Fig.  14  illustrates  a 
method  of  dimensioning  the  dovetail  slide  shown  in  Fig.  13 
which  avoids  compound  tolerances.  The  dimensions  that  con- 
trol the  position  of  the  angular  flanks  are  given  to  the  sharp 


INTERCHANGEABLE   MANUFACTURING 


corners  at  the  top  of  the  dovetail.  The  tolerances  on  these 
dimensions  limit  the  permissible  variation  of  these  angular 
flanks.  The  angle  is  given  as  a  flat  dimension.  As  is  evident 
from  the  functional  drawing,  Fig.  13,  the  bearing  surface  A 
and  these  angular  flanks  are  the  essential  functional  surfaces 
of  this  dovetail.  All  other  surfaces  are  clearance  surfaces,  as 
should  be  apparent  from  the  extent  of  the  tolerances,  Fig.  14, 
even  though  the  functional  drawing  were  not  available.  Fig. 
15  shows  graphically  the  applications  of  the  tolerances  given 
in  Fig.  14.  The  full  lines  represent  the  maximum  metal  con- 


Machlncry 


Fig.  15.    Graphical  Illustration  of  Application  of  Tolerances 

ditions,  while  the  dotted  lines  indicate  the  minimum  metal 
conditions. 

Compound  Tolerances.  The  dimensioning  of  tapered  plugs 
and  holes  introduces  a  somewhat  similar  problem  which  will 
result  in  a  condition  of  compound  tolerances  if  great  care  is  not 
exercised.  Fig.  16  shows  such  a  tapered  hole  as  it  is  usually 
dimensioned.  This  method  of  dimensioning  is  wrong,  as  it 
creates  a  condition  of  compound  tolerances.  With  these  di- 
mensions, it  is  impossible  to  determine  what  final  result  is  re- 
quired, since  there  are  so  many  possible  combinations.  It  is 
evident  that  as  the  diameter  of  either  the  large  or  the  small  hole 
varies,  the  taper  will  change.  This  makes  an  uncertainty  about 
the  reamers,  as  these  tools  have  a  fixed  taper.  If  we  assume 
that  the  taper  is  constant,  questions  will  be  raised  as  to  which 
combination  of  limits  to  employ  to  establish  the  taper.  If 
we  further  assume  that  the  basic  dimensions  are  to  be  used 


COMPONENT  DRAWINGS 


59 


for  this  purpose,  the  next  question  will  be  whether  this  taper, 
considered  as  a  constant,  is  required  to  remain  in  the  position 
indicated  by  the  dimension  i.ooo  inch  -o'S^>  under  all  con- 
ditions, or  whether  it  can  also  vary  in  addition  by  the  amount 
resulting  from  the  variations  in  diameters.  Also  a  tolerance  is 
given  on  the  length  of  the  taper.  This  is  entirely  meaningless. 
It  cannot  be  measured  readily  even  with  an  elaborate  laboratory 
equipment  and  there  is  no  use  for  this  tolerance  in  the  course  of 
manufacture.  With  a  fixed  taper,  the  variation  in  this  length  is 
controlled  absolutely  by  the  relative  size  of  the  holes.  All  in 
all,  as  the  drawing  stands,  it  is  a  puzzle  without  any  solution. 


r 


rnA'-f-  0.010., 
500  —  0.000  " 


Machinery 


Fig.  16.    Incorrect  Method  of  dimensioning  Tapered  Hole 

We  will  assume  that  the  intent  of  Fig.  16  is  to  indicate  a 
constant  taper  with  a  tolerance  of  -fo.oio  inch  in  regard  to 
its  position.  Fig.  17  shows  the  correct  method  of  dimensioning 
such  a  surface  to  maintain  such  a  condition.  An  arbitrary  point 
is  taken  on  the  taper  and  a  fixed  dimension  given  for  its  diameter 
at  that  point.  The  location  of  this  fixed  diameter  is  dimensioned 
with  the  tolerance.  Three  methods  of  dimensioning  this  taper 
are  shown.  Either  of  the  first  two,  A  or  B,  is  preferable  to  the 
third,  C,  because  any  reference  figures  desired  can  be  readily 
computed  from  them  without  recourse  to  trigonometry  or  any 
tables  or  handbooks. 

Fig.  17  gives  the  manufacturer  definite  information  which  he 
can  use  and  which  he  can  use  in  only  one  way.  The  tolerances 


6o 


INTERCHANGEABLE   MANUFACTURING 


given  on  each  dimension  apply  only  to  the  specific  surface 
in  question.  No  tolerance  can  be  given  on  the  diameter  of 
the  taper  nor  on  the  angle  without  introducing  compound 
tolerances  again,  with  resultant  confusion.  The  permissible 
variations  on  this  tapered  surface  are  fully  established  by 


TAPER  0.400  PER  INCH 

B 

THREE  METHODS  OF  DIMENSIONING  ANGLE 


Machinery 


Fig.  17.     Correct  Method  of   dimensioning  Tapered  Hole 


Machinery 


Fig.  18.    Graphical  Illustration  of  Application  of  Tolerance 

the  tolerance  given  on  its  location.  Fig.  18  shows  graphically 
the  maximum  and  minimum  metal  conditions  established  by  the 
dimensions  and  tolerances  given  in  Fig.  17.  It  will  be  noted 
that  a  parallel  zone  for  the  permissible  variations  has  been 
established  on  every  surface.  When  this  has  been  accomplished, 
no  further  tolerances  should  be  given. 


COMPONENT  DRAWINGS 


6l 


The  method  of  dimensioning  a  taper  shown  in  Fig.  17  usually 
meets  with  more  or  less  opposition  from  the  shop  men.  The 
objection  is  raised  that  more  dimensions  are  necessary  in  order 
to  make  up  the  proper  reamers,  etc.  Although  the  needed  di- 
mensions can  be  readily  computed,  it  is  desirable  to  reduce 
the  amount  of  such  computations  in  the  shop  as  much  as  possi- 
ble. This  objection  can  be  eliminated  in  several  ways.  First, 
if  a  drawing  is  made  for  the  reamers,  all  the  additional  checking 
dimensions  can  appear  on  these  drawings.  Second,  if  opera- 
tion drawings  are  provided,  these  dimensions  would  appear 


BEARING  OR  FUNCTIONAL  SURFACES 


CLEARANCE 

MIN.  INTERFERENCE  FOR  KEY  0.002" 
MAX.  INTERFERENCE  FOR  KEY  O.OOS" 


Machinery 


Fig.  19.    Functional  Requirements  for  a  Taper  Key 

there.  Third,  if  neither  of  the  two  foregoing  practices  is  adopted, 
the  required  dimensions  may  appear  on  the  component  drawing 
in  parentheses,  and  may  be  marked  " Basic"  or  "Reference." 
It  should  be  clearly  understood,  however,  that  such  dimensions 
are  supplementary  and  apply  only  in  connection  with  the  other 
basic  dimensions  given.  No  tolerances  should  under  any  cir- 
cumstances be  given  on  such  reference  figures.  As  far  as  possi- 
ble they  should  be  eliminated  from  the  drawing. 

Dimensioning  Force  Fits.  The  dimensioning  of  a  taper  key 
and  its  seat  offers  a  very  instructive  example.  In  this  case,  we 
have  a  drive  fit  so  that  instead  of  clearances  we  must  concern 
ourselves  with  the  establishment  of  the  proper  interferences. 
Fig.  19  illustrates  such  a  key  and  its  seat,  The  functional  con- 


62 


INTERCHANGEABLE   MANUFACTURING 


ditions  to  be  maintained  demand  that  we  have  always  an  inter- 
ference of  at  least  0.002  inch  and  never  have  a  greater  interference 
than  0.008  inch.  The  illustration  shows  clearance  at  those  points 
at  which  no  bearing  is  required.  Often,  however,  we  find  draw- 
ings for  such  functional  conditions  specifying  fits  on  all  surfaces. 
Such  conditions  add  nothing  to  the  strength  or  effectiveness  of 
the  construction  but  entail  unnecessary  refinement  in  the  manu- 
facture of  the  detailed  parts  with  a  correspondingly  increased 


«   II 

=>=>  00 

5 


f-H 

2 


4-0.000. 

-o.oio" 


TAPER   0.625  INCH 

—PER  FOOT  2>5001 


TAPER  0.625  INCH 
PER  FOOT 


_JL 


Machinery 


Fig.  20.    Incorrect  Dimensioning  and  Design  of  Details 
shown  in  Fig.  19 

cost.  Fig.  20  illustrates  the  details  of  such  a  condition  dimen- 
sioned in  a  very  common  manner.  This  method  of  dimensioning 
is  wrong.  The  key  in  this  sketch  violates  the  first,  second,  third, 
and  fifth  laws  of  dimensioning;  the  slide  violates  the  second, 
third  and  fifth  laws;  while  the  dimensioning  on  the  seat  vio- 
lates the  first,  second,  third,  and  fifth  laws.  With  the  dimen- 
sions given  as  they  are,  it  is  impossible  to  specify  tolerances  that 
will  insure  the  required  functional  conditions  unless  we  reduce 
each  tolerance  to  a  fraction  of  a  thousandth.  The  dimensions 
and  the  tolerances  as  they  stand  permit,  in  some  cases,  the  key 


COMPONENT  DRAWINGS 


to  be  tight  in  the  slide  and  loose  in  the  seat  of  the  slide.  In 
other  cases,  the  reverse  is  true.  Parts  made  to  the  basic  di- 
mensions will  have  a  fit  on  all  surfaces. 

Fig.  21  shows  these  parts  dimensioned  in  accordance  with 
the  laws  of  dimensioning.  It  will  be  noted  that  with  parts 
made  to  the  basic  dimensions,  the  key  will  be  driven  home  to 
its  head,  with  an  interference  on  the  bearing  surfaces  specified 
of  0.002  inch.  The  direction  of  the  tolerances  on  every  dimen- 


TAPER  0.625  INCH  PER  FOOT 
(SIDES  OF  SLOT 
MAY  BE  PARALLEL) 


do 

TAPER  0.625  INCHL<4o-> 
PER  FOOT 


Machinery 


Fig.  21.    Correct  Dimensioning  and  Design  of  Details  shown  in 
Fig.  19 

sion  affecting  these  bearing  surfaces  is  such  that  this  interference 
is  increased  as  the  sizes  of  the  parts  vary  from  the  basic  dimen- 
sions. Under  maximum  metal  conditions,  the  bottom  of  the 
key  will  be  flush  with  the  bottom  of  the  slide  with  an  inter- 
ference of  0.008  inch  on  the  functional  surfaces.  It  should 
be  noted  that  by  giving  the  dimensions  in  this  manner,  the 
required  conditions  are  always  maintained,  while  the  manu- 
facturing tolerances  are  greatly  increased.  Both  slots  are  made 
with  parallel  sides  to  facilitate  machining.  Fig.  21  offers  a 
good  example  of  the  application  of  the  fifth  law  of  dimen- 


INTERCHANGEABLE    MANUFACTURING 


sioning.  This  illustration  should  be  carefully  studied  and  com- 
pared with  Fig.  20.  Note,  in  particular,  the  ease  of  checking 
the  functional  conditions  in  Fig.  21  as  contrasted  with  the  diffi- 
culty and  confusion  which  arises  if  we  attempt  to  determine  the 
possible  combinations  permitted  in  Fig.  20.  Note  also  how  the 
relative  extent  of  the  tolerances  specified  in  Fig.  21  calls  atten 
tion  to  the  essential  functional  surfaces.  These  same  relative 
conditions  exist  between  any  drawings  that  are  dimensioned 
without  careful  study  as  compared  with  those  which  are  ra- 
tionally and  logically  dimensioned.  No  attempt  has  been  made 


0.560  RAD. 


Machinery 


Fig.  22. 


Sketch  showing  Satisfactory  Method  of  specifying 
Tolerance  on  Contours 


in  this  example  to  express  any  dimensions  other  than  those  which 
affect  the  taper  key  and  its  seat.  The  example  given  in  Fig.  14 
shows  the  proper  dimensioning  of  the  dovetail  slide. 

Dimensioning  of  Profile  Surfaces.  The  dimensioning  of  con- 
tours with  tolerances  introduces  still  another  problem.  To  give 
tolerances  on  the  various  dimensions  which  establish  the  basic 
contour  inevitably  introduces  compound  tolerances.  On  the 
other  hand,  it  is  often  impossible  to  resolve  such  composite  sur- 
faces into  elementary  ones  for  the  purposes  of  dimensioning  and 
checking,  because  their  dimensions  and  relative  locations  are 
inseparable.  Fig.  22  illustrates  one  satisfactory  solution  of 
this  problem.  The  basic  dimensions  of  the  profile  are  given 
without  tolerances.  A  dotted  line  is  drawn  parallel  to  the  basic 


COMPONENT  DRAWINGS 


contour  which  indicates  the  direction  of  the  tolerance.  A 
dimension  is  given  between  the  full  (or  basic)  outline  and  this 
dotted  line  which  specifies  the  extent  of  the  tolerances.  This 
method  of  dimensioning  gives  definite  information  which  can 
be  used  directly  in  the  manufacturing  departments. 

Dimensioning  of  Holes.  The  dimensioning  of  the  location  of 
holes  with  tolerances  is  a  most  difficult  problem.  These  dimen- 
sions are  usually  given  to  the  centers  of  the  holes  and  define 
neither  male  nor  female  surfaces.  They  must  be  used  in  con- 
junction with  the  diameters  of  the  holes,  thus  establishing  a 
composite  surface  condition.  The  introduction  of  tolerances  on 


Machinery 


Fig.  23.    Diagram  illustrating  Conditions  met  with  in  measuring 
Holes 

these  dimensions  of  location  immediately  will  produce  com- 
pound tolerances. 

We  might  dimension  them  as  shown  in  Fig.  23  by  giving  one 
dimension  to  the  inside  edges  of  the  holes  (which  is  a  male 
dimension),  another  to  the  outside  edges  of  the  holes  (a  female 
dimension),  and  eliminate  the  dimension  for  diameter.  This 
would  give  us  a  better  opportunity  of  applying  the  five  laws  of 
dimensioning  in  a  similar  manner  to  that  employed  for  elemen- 
tary surfaces.  However,  this  would  prove  unsatisfactory  in 
practice  because  it  does  not  give  directly  the  information  which 
is  of  most  value  in  the  shop  —  namely,  the  diameters  of  the 
holes  and  the  center  distances. 


66 


INTERCHANGEABLE   MANUFACTURING 


No  rules  can  safely  be  given  for  dimensioning  the  location  of 
holes  in  which  the  permissible  variations  are  distinctly  expressed, 
unless  the  required  functional  conditions  are  duly  considered. 
The  following  examples  give  possible  solutions  of  a  few  of  these 
problems.  If  these  solutions  are  accepted,  the  corresponding 
interpretations,  expressed  in  terms  of  inspection  gage  require- 
ments, must  also  be  accepted.  For  the  first  example,  we  will 
take  the  base  for  a  bracket  and  its  pad  on  a  frame  illustrated 
in  Fig.  24.  We  will  assume  that  the  position  of  this  bracket  on 
the  frame  is  important  and  must  be  held  as  closely  as  manu- 
facturing conditions  will  permit.  We  will  assume  also  that  the 


DIAMETER  OF  STUD    0.746+°'°°° 


Machinery 


Fig.  24.    Methods  of  dimensioning  the  Location  of  Holes 

jigs  from  which  these  holes  are  drilled  locate  the  parts  on  the 
finished  surfaces  from  which  the  dimensions  are  given. 

Causes  of  Variation  in  Manufacture.  Variations  of  locations 
in  manufacture  develop  from  three  main  causes:  First,  from  a 
fixed  error  in  the  jig;  second,  from  a  difference  in  size  between 
the  drill  and  its  bushing  in  the  jig;  and  third,  from  improper 
location  of  the  parts  in  the  jigs.  Variations  occurring  because 
of  the  first  cause  will  affect  the  locations  of  the  holes  both  in 
relation  to  each  other  and  to  their  locating  surfaces.  Varia- 
tions because  of  the  second  cause  will  have  similar  effects  to 
those  developing  from  the  first  cause.  Variations  because  of 
the  third  cause  will  affect  only  the  location  of  all  holes  as  a  unit 
from  the  locating  surfaces.  Thus,  with  these  problems,  there 
are  always  composite  variations  with  which  to  contend.  The 


COMPONENT   DRAWINGS 


surfaces  involved  are  always  composite,  and  a  condition  of 
compound  tolerances  is  always  present. 

If  precision,  rather  than  absolute  accuracy,  is  the  main  con- 
sideration, and  if  these  locations  in  the  two  jigs  check  with  each 
other,  the  variations  due  to  the  first  cause  may  be  disregarded, 
provided  that  the  gages  which  check  these  locations  are  made  to 
agree  with  the  jigs. 

The  variations  due  to  the  second  cause  may  be  reduced  to 
comparatively  small  amounts  by  closely  maintaining  the  rela- 
tive diameters  of  the  drills  and  their  bushings.  This  naturally 
involves  a  somewhat  increased  maintenance  cost  of  the  equip- 
ment. The  extent  of  the  variations  due  to  the  third  cause 


I 

_i_ 

— 

r 

^                   r 

^ 

i   a         ? 
3.000 

!  -rr 

4  T 

^ 
f. 

J                    ^ 

^                   r 

\) 
*\ 

k?**II  

\, 

j                   \. 

\) 

2.000" 

J_ 

__. 

"^li.odd 

h- 

n 

5.000  > 

•<  3.000--^ 

Machinery 

Fig.  25.     Functional  Gage  for  Part  shown  in  Fig.  24 

depends  upon  the  design  of  the  jigs  and  the  care  exercised  by 
the  operator.  In  general,  the  third  cause  is  responsible  for  the 
largest  amount  of  variation. 

The  locations  in  Fig.  24  are  given  without  tolerances,  yet  the 
drawing  should  not  be  interpreted  to  mean  that  no  variations 
are  permissible.  The  minimum  clearance  between  the  studs 
and  the  holes  is  0.004  mcn-  This  clearance  is  provided  to  allow 
for  the  variations  in  their  locations.  Therefore,  this  clearance 
should  be  considered  in  testing  the  locations  of  these  holes. 

Gages  for  Checking  Location  of  Holes.  The  inspection  gage 
for  testing  these  locations  would  be  a  functional  gage  which 
invades  this  minimum  clearance.  There  are  two  conditions  to 
be  considered  here  which  affect  the  amount  of  the  minimum 


68 


INTERCHANGEABLE   MANUFACTURING 


clearance  that  may  be  used  on  the  gage.  If  the  studs  used  are 
loose,  individual  pieces  which  pass  through  both  parts  and  are 
bolted  or  riveted  at  assembly,  the  functional  gage  may  utilize 
the  entire  minimum  clearance.  On  the  other  hand,  if  the  studs 
are  first  driven  or  riveted  into  one  member,  these  functional 
gages  could  invade  the  minimum  clearance  not  over  fifty  per 
cent.  In  either  case,  it  is  possible  to  make  a  single  gage  which 
will  check  both  parts. 

We  will  first  consider  the  functional  gage  to  check  the  first 
of  the  above  conditions.  This  gage  would  consist  of  a  plate 
with  four  pins,  as  shown  in  Fig.  25.  It  checks  both  the  loca- 
tions of  the  four  holes  relative  to  each  other  and  the  location  of 


\< 5.000" — 


Machinery 


Fig.  26.    Another  Type  of  Functional  Gage  for  Part  shown  in  Fig.  24 

the  group  from  the  edges  of  the  part.  The  locations  of  the  pins 
are  identical  with  the  corresponding  basic  dimensions  given  on 
the  component  drawings.  The  diameter  of  the  pins  is  0.746 
inch  (basic  diameter  of  hole  minus  minimum  clearance).  The 
gage  must  always  enter  all  four  holes  on  the  part.  When  the 
gage  is  held  against  the  upper  edge  of  the  holes,  the  lower  edge 
of  the  gage  must  not  project  below  the  lower  edge  of  the  com- 
ponent. When  held  against  the  lower  edge  of  the  holes,  the 
lower  edge  of  the  gage  must  not  be  above  the  lower  edge  of  the 
part.  This  checks  the  vertical  position  of  the  holes.  The  hori- 
zontal locations  are  checked  in  a  similar  manner.  The  diameters 
of  the  holes  are  checked  as  elementary  surfaces  with  limit  plug 
gages  made  to  the  specified  limits. 


COMPONENT   DRAWINGS 


69 


Thus,  although  both  the  drawings  and  the  gages  are  made 
to  flat  dimensions,  a  tolerance  on  all  positions  of  the  holes  has 
been  established.  If  their  relative  locations  were  perfect,  under 
maximum  metal  conditions  of  the  various  holes,  a  variation  of 
0.002  inch  either  way  would  be  permitted  on  their  location  from 
the  edges  of  the  parts.  If  the  various  holes  were  made  to  the 
maximum  limits,  this  variation  could  be  0.005  mcn  either  way. 
On  the  other  hand,  if  the  position  of  these  holes  as  a  unit  were 
perfect  in  regard  to  the  edges  of  the  components,  a  variation  of 
0.004  inch  would  be  permitted  on  their  relative  locations  under 
maximum  metal  conditions,  with  a  correspondingly  increasing 


T7 

2.000^ 

0  7fM)'  +  °'006i» 

iMpu_0  000» 

0  7  V»  +0.006" 
IMOU_O<OOO» 

-t 

••» 

c 

i             C 

^ 

( 

Y        ( 

> 

>• 

}         t 

^             r 

J 

^ 

t 

2.0JX)"s 

> 

r 

J                    \. 

^                / 

JL 

?         ^ 
f 

J 

2.000" 

I 

\. 

J                    t 

—  >ti.odo 

DIAMET 

<-  —  4.<XXh--*| 

ER  OF  STUD   0.746  Jl' 

4 

W;           f—  S.OOO---* 

.004 

<  ^ooo'^  —  >j 

Machinery 

Fig.  27.     Method  of  dimensioning  Location  of  Holes  without 
Tolerances 

tolerance  as  the  parts  approach  minimum  metal  conditions. 
This  would  amount  to  o.oio  inch  at  the  extreme  minimum  metal 
condition.  Inasmuch  as  variations  will  develop  in  both  types 
of  locations,  all  that  is  not  consumed  by  one  is  available  for 
the  other. 

We  will  now  consider  the  functional  gage  to  check  the  con- 
dition where  the  studs  are  rigidly  fastened  to  one  of  the  parts. 
A  gage  for  this  purpose  would  be  similar  to  the  one  shown  in 
Fig.  25  except  that  it  would  contain  four  holes  instead  of  four 
pins.  Such  a  gage  is  shown  in  Fig.  26.  Four  plugs  would  be 
used  with  this  gage  for  testing  the  locations  of  the  holes  in  one 
piece,  while  the  holes  in  the  gage  would  go  over  the  studs  fas- 
tened to  the  companion  part.  The  diameter  of  the  holes  in  this 


INTERCHANGEABLE   MANUFACTURING 


gage  and  also  of  the  plugs  is  0.748  inch  (basic  diameter  of  hole 
minus  one-half  minimum  clearance  or  basic  diameter  of  stud 
plus  one-half  minimum  clearance).  This  gage  would  be  used 
in  exactly  the  same  manner  as  the  first.  The  permissible  varia- 
tions under  maximum  metal  conditions  of  the  holes  and  studs 
would  be  but  one-half  that  permitted  in  the  first  case.  As  these 
holes  and  studs  approach  minimum  metal  conditions,  the  per- 
missible variations  in  location  would  increase  in  the  same  man- 
ner and  extent  as  in  the  first  case. 

In  Fig.  24,  the  locating  dimensions  are  given  from  common 
locating  surfaces  in  each  direction.     They  could  be  given  as 


ONE  METHOD  OF  DIMENSIONING 


IC 

b1            ( 

h 

>- 

» 

( 

P            ^ 

"^                    C 

±0.002" 

c 

J                   C 

1              f 

J 

3.000  ±0-002  j«— 
<  7.000"±  0.002-  ;- 

DIAMETER  OF  STUD=  0.746  Ij^w  SECOND  METHOD  OF  DIMENSIONING 


Machinery 


Fig.  28.    Two  Methods  of  Dimensioning  with  Tolerance  which 
maintain  Identical  Conditions 

shown  in  Fig.  27.  As  long  as  no  tolerances  are  expressed,  the 
method  most  convenient  for  the  shop  is  best. 

Expressed  Tolerances  on  Location  of  Holes.  If  expressed 
tolerances  for  locations  of  holes  are  insisted  upon,  it  is  impossi- 
ble to  avoid  compound  tolerances.  Then  an  arbitrary  method 
of  interpretations  must  be  promulgated  to  prevent  continual 
argument  and  misunderstanding.  Fig.  28  illustrates  two 
methods  of  indicating  such  tolerances.  We  assume  that  the 
functional  conditions  are  identical  with  those  previously  dis- 
cussed in  the  first  case  of  Fig.  24.  This  is  one  example  where 
the  mean  size  is  the  proper  basic  dimension,  and  tolerances  apply 
equally  plus  and  minus. 

In  order  to  establish  the  sizes  of  inspection  gages  we  must 
consider  the  tolerances,  instead  of  minimum  clearances.  Re- 


COMPONENT   DRAWINGS  71 

ferring  to  Fig.  23,  an  inspection  gage  to  check  the  relative 
locations  of  the  holes  must  be  made  to  the  maximum  dimension 
A  and  the  minimum  dimension  B.  In  the  horizontal  direction, 
the  maximum  limit  of  A  is  equal  to  the  maximum  center  distance 
(4.004  inches)  minus  the  minimum  diameter  of  the  hole  (0.750 
inch)  which  amounts  to  3.254  inches.  The  minimum  limit  of 
B  in  the  same  direction  is  equal  to  the  minimum  center  distance 
(3.996  inches)  plus  the  minimum  diameter  of  the  hole  (0.750 
inch)  which  is  equal  to  4.746  inches.  The  difference  between  A 
maximum  and  B  minimum  gives  double  the  diameter  of  the 
pins  on  the  inspection  gage,  which  amounts  to  1.492  inches. 
The  diameter  of  these  pins  is  therefore  0.746  inch,  and  the 
inspection  gage  for  the  relative  location  of  the  holes  is  identical 
with  the  one  shown  in  Fig.  25. 

In  like  manner,  we  must  consider  the  location  of  these  holes 
from  the  edge  of  the  components.  For  simplicity  in  notation, 
call  the  dimension  from  the  lower  edge  of  the  piece  (in  Fig.  28) 
to  the  upper  edge  of  the  circumference  of  the  lower  left-hand 
hole,  C.  Call  the  distance  from  the  lower  edge  of  the  piece  to 
the  bottom  edge  of  the  hole,  D.  On  the  gage,  evidently,  C  must 
be  minimum,  while  D  must  be  maximum.  The  minimum  di- 
mension of  C  is  equal  to  0.998  inch  plus  half  the  minimum 
diameter  of  the  hole  (0.375  mcn)  which  amounts  to  1.373  inches. 
The  maximum  dimension  D  is  equal  to  1.002  inches  minus  half 
the  minimum  diameter  of  the  hole,  which  equals  0.627  mcn- 
The  diameter  of  the  pins  in  the  gage  is  equal  to  the  difference 
between  C  and  D,  which  equals  0.746  inch.  Therefore,  the  gage 
shown  in  Fig.  25  applies  to  both  Fig.  24  and  Fig.  28.  Or,  to 
put  it  in  other  words,  Fig.  24  and  Fig.  28  express  the  same 
information. 

Therefore,  in  those  cases  where  no  tolerances  are  given  for 
center  distance  (and  this  applies  equally  to  locations  of  holes  or 
grooves  or  slots,  etc.),  the  minimum  clearance  must  be  analyzed, 
and  utilized  accordingly  to  determine  the  inspection  gage  re- 
quirements, and  a  suitable  minimum  clearance  must  be  pro- 
vided to  allow  for  the  inevitable  variation  in  these  dimensions. 
On  the  other  hand,  where  tolerances  are  specified  on  such  di- 


INTERCHANGEABLE   MANUFACTURING 


mensions,  these  must  be  analyzed  and  applied  accordingly,  to 
establish  the  inspection  gage  dimensions,  and  the  minimum 
clearance  between  the  holes  and  studs  must  be  sufficient  to 
prevent  interferences.  In  either  case,  the  basic  dimensions 
should  be  identical,  and  the  inspection  gages  would  also  be 
identical.  For  conditions  such  as  those  described,  experience 
will  teach  that  the  safest  plan  is  to  eliminate  the  tolerances  from 
the  component  drawings. 

The  above  interpretations  of  drawings  are  arbitrary  to  a  cer- 
tain extent.  It  would  be  possible  to  demonstrate  that  under 
certain  combinations  of  conditions,  the  exact  letter  of  the  com- 
ponent drawings  would  be  violated.  This  is  inevitable  in  this 


-J 


—  H     r  —  4<00°''  —  H 

1  1.000" 


±0.040" 
DIAMETER  OF  STUDS     0.746"+°-°OOjJ 


.    r 

y 

i 

H           r 

> 

N 

"> 

V 

^ 

J                     V. 

•>!               r 

.« 

C 

J               t 

7 

^. 

<—  „  > 
3.000  ±0.040" 

<  —  4.000"  —  * 

M( 

cftinc 

Fig.  29.    Another  Method  of  Dimensioning  with  Tolerance  Part 
shown  in  Fig.  24 

connection.    As  stated  before,  if  these  solutions  are  accepted, 
the  corresponding  interpretations  must  also  be  accepted. 

Tolerances  for  "  Group  "  Locations.  The  same  parts,  but 
with  different  functional  conditions,  will  now  be  considered. 
Naturally,  the  holes  in  the  companion  parts  must  line  up  suffi- 
ciently to  enable  the  studs  to  pass  through.  Therefore,  the 
importance  of  the  location  of  these  holes  in  relation  to  each  other 
is  constant.  We  will  assume,  however,  that  in  this  case  the 
position  of  the  bracket  on  the  frame  is  unimportant.  The  only 
method  of  indicating  this  condition  that  will  be  consistent  with 
the  general  practice  of  dimensioning  discussed  heretofore  will 
be  to  express  a  tolerance.  This  is  done  in  Fig.  29.  No  toler- 
ances are  shown  in  this  sketch  on  the  dimensions  controlling  the 
relative  locations  of  the  holes  to  each  other. 


COMPONENT   DRAWINGS 


73 


The  interpretation  of  this  drawing  is  that  a  variation  of  0.040 
inch,  plus  and  minus,  over  and  above  that  allowed  by  the  mini- 
mum clearance  between  studs  and  holes  is  permitted  on  the 
location  of  the  holes  as  a  group.  The  inspection  gage  for  test- 
ing this  is  shown  in  Fig.  30.  This  gage  differs  from  that  shown 
in  Fig.  25  only  in  the  addition  of  the  steps  on  the  edges  which 
check  the  additional  tolerance.  It  is  used  as  follows:  When 
the  gage  is  held  against  the  upper  edges  of  the  holes,  the  mini- 
mum lower  step  of  the  gage  must  not  extend  below  the  lower 
edge  of  the  part.  When  the  gage  is  held  against  the  lower  edge 
of  the  holes,  the  lower  maximum  step  must  not  be  above  the 
lower  edge  of  the  part.  The  horizontal  locations  are  checked 


Machinery 


Fig.  30.    Functional  Gage  for  the  Part  shown  in  Fig.  29 

in  a  similar  manner.  If  the  functional  conditions  permit  a  liberal 
variation  in  one  direction  (say,  horizontal)  but  not  in  the  other 
direction  (vertical),  a  combination  of  the  methods  of  dimen- 
sioning and  checking  meets  the  situation. 

Conditions  arise  where  the  locations  of  holes  must  be  estab- 
lished and  checked  from  other  points  than  a  flat  surface.  This 
often  requires  quite  elaborate  fixture  gages.  A  full  understand- 
ing of  the  preceding  principles  and  a  careful  study  of  the  par- 
ticular conditions  will  point  the  way  to  a  consistent  solution  of 
the  problem.  The  present  space  is  not  sufficient  to  go  into  the 
subject  in  greater  detail.  Simple  examples  have  been  purposely 
selected  to  indicate  and  illustrate  the  general  principles  involved. 


74 


INTERCHANGEABLE   MANUFACTURING 


The  preceding  examples  involve  maintaining  the  relative 
position  of  several  holes  with  each  other  in  addition  to  the  loca- 
tion of  a  group  as  a  whole.  In  those  cases  where  only  a  single 
hole  is  involved  which  must  maintain  its  position  in  relation  to 
elementary  surfaces,  the  problem  is  simple.  In  most  cases  it 
can  be  solved  by  the  application  of  methods  previously  discussed 
for  elementary  surfaces.  In  other  cases,  the  functional  require- 
ments may  be  such  as  to  demand  a  functional  gage  similar  to 
those  shown  in  Figs.  25  and  30. 

Concentricity  and  Alignment.  The  expression  of  permissible 
variations  in  concentricity  and  alignment  introduces  another 
difficult  problem.  The  succeeding  examples  offer  one  solution. 


U'  "  I 
3.500  ±S:«£' — *j               h-2.0l6'i°;^(;->| 

h- 


Machinery 


Fig.  31.    Methods  of  dimensioning  Holes  and  Studs  to  fit 

As  in  the  case  of  the  locations  of  holes,  if  these  solutions  are 
accepted,  the  corresponding  interpretations,  expressed  in  terms 
of  inspection  gage  requirements,  must  also  be  accepted. 

We  will  assume  that  the  stud  shown  in  Fig.  31  must  always 
assemble  into  the  hole  shown  in  the  same  sketch.  In  the  process 
of  manufacture,  a  certain  amount  of  eccentricity  will  develop. 
If  we  attempt  to  give  on  the  component  drawings  the  permis- 
sible eccentricity  in  every  case,  the  drawing  will  become  more 
and  more  complicated.  The  more  complicated  the  drawings 
become,  the  greater  the  possibility  of  undetected  errors.  With 
the  following  interpretations  of  the  drawings,  the  conditions  of 
eccentricity  are  almost  automatically  covered. 


COMPONENT   DRAWINGS 


75 


There  is  a  minimum  clearance  on  diameters  of  0.004  inch 
between  the  parts  shown  in  Fig.  31.  Inspection  gages  to  test 
the  concentricity  of  these  parts  are  functional  gages  and  invade 
this  minimum  clearance  to  a  fair  amount.  In  general,  this 
should  not  be  over  fifty  per  cent.  The  lengths  of  the  studs  and 
depths  of  the  holes  do  not  enter  into  this  discussion.  They  are 
elementary  surfaces  which  should  be  readily  maintained  and 
checked.  The  gages  for  testing  the  concentricity  of  these  parts 


Machinery 


Fig.  32.    Functional  Gages  for  Part  shown  in  Fig.  31 

are  shown  in  Fig.  32.  It  will  be  noted  that  the  diameters  invade 
the  minimum  clearance  by  an  amount  equal  to  fifty  per  cent. 
A  somewhat  similar  condition  which  involves  the  alignment  of 
slides,  or  profile  grooves  and  tongues,  has  been  previously  dis- 
cussed in  connection  with  Fig.  10.  Functional  gages  for  such 
conditions  are  shown  in  Fig.  12. 

Occasionally,  the  situation  arises  where  a  sub-assembly  as  a 
whole  must  meet  such  conditions  as  are  described  above.    This 


76  INTERCHANGEABLE   MANUFACTURING 

may  entail  individual  tests  for  concentricity  or  alignment  on 
individual  component  parts  of  the  sub-assembly.  The  mini- 
mum clearances  must,  therefore,  be  subdivided  proportionately. 
In  such  cases,  it  is  good  practice  to  include  on  the  component 
drawing  an  outline  with  the  dimensions  of  the  functional  gage 
required  to  test  the  conditions  of  concentricity  or  alignment. 
This  practice  will  eliminate  many  arguments  in  the  course  of 
future  production. 

Gears.  Gear  teeth  offer  another  problem  of  composite  sur- 
faces. In  general  the  tolerances  on  the  tooth  forms  can  best 
be  given  by  specifying  the  permissible  amount  of  backlash 
between  the  pair.  No  tolerances  should  ever  be  given  on  pitch 
diameters  of  gears.  Specifying  a  limit  on  the  backlash  makes 
it  possible  to  eliminate  all  compound  tolerances.  Furthermore, 
the  most  effective  inspection  of  the  gears  is  obtained  by  measur- 
ing this  backlash  with  the  gears  at  a  fixed  center  distance.  All 
the  foregoing  examples  are  comparatively  simple  ones.  They 
should,  however,  be  sufficient  to  indicate  the  manner  in  which  a 
component  drawing  with  tolerances  should  be  dimensioned. 

As  stated  previously,  it  is  seldom  possible  at  the  very  start 
to  collect  and  record  on  the  component  drawings  all  of  the  de- 
tailed information  which  belongs  there.  The  development  of 
tools,  gages,  and  other  equipment  and  the  final  solution  of  many 
of  the  manufacturing  problems  will  make  apparent  omissions 
and  errors.  Therefore,  the  component  drawings  should  not  be 
considered  as  complete  until  the  product  is  actually  being  pro- 
duced in  strict  accordance  with  them.  This  requires  that  the 
designer,  responsible  for  the  accuracy  of  the. drawings,  keep  in 
close  touch  with  both  the  designers  of  the  manufacturing  equip- 
ment and  the  various  manufacturing  departments  in  order  to 
keep  these  component  drawings  up  to  date. 


CHAPTER  VI 
PRACTICE    IN    MAKING    COMPONENT    DRAWINGS 

As  a  practical  and  specific  illustration  of  the  principles  gov- 
erning the  dimensioning  of  component  drawings  set  forth  in 
the  preceding  chapter,  a  small  unit  assembly  showing  the  per- 
cussion firing  mechanism  for  a  large  cannon  is  taken  as  an 
example.  This  particular  mechanism  is  chosen  because  it  is 
composed  of  a  small  number  of  parts;  also  because  it  contains 
several  examples  of  comparatively  unusual  conditions.  In 
studying  the  various  component  or  detail  drawings  to  be  re- 
ferred to,  the  relation  between  the  methods  of  dimensioning, 
the  tolerances  and  clearances  specified,  and  the  functional  re- 
quirements of  each  part  should  be  carefully  considered. 

Drawing  of  Firing  Mechanism  Assembled.  The  assembly 
of  this  mechanism  is  shown  in  Fig.  i.  The  operation  is  as  follows: 
The  firing  mechanism  container  assembled  must  be  withdrawn 
before  the  breech  of  the  cannon  can  be  opened,  and  cannot  be 
replaced  until  the  breech  is  closed.  (The  safety  mechanism  con- 
trolling this  is  not  shown  on  this  drawing.)  While  this  assem- 
bled container  is  being  withdrawn,  a  primer  A  is  inserted  in  the 
primer  extractor.  This  primer  is  held  in  place  by  the  pressure 
of  the  firing  pin  guide  spring  acting  against  the  firing  pin  guide 
B.  After  the  breech  has  been  closed  again,  the  container  C  with 
the  primer  is  inserted  into  the  housing  D  and  screwed  home  by 
hand.  The  primer  must  seat  tightly  on  the  sharp  taper  in  the 
spindle  plug  E.  A  lanyard  is  attached  to  the  striker  F  with  a 
connection  that  slips  off  when  the  end  of  the  striker  is  withdrawn 
beyond  the  end  of  the  container  cover  G,  thus  allowing  the 
striker  to  move  forward  at  the  proper  moment  under  the  im- 
pulse of  the  firing  spring  H.  The  firing  pin  transmits  the  blow 
of  the  striker  to  the  primer,  thus  detonating  it  and  igniting  the 
charge  in  the  cannon. 

77 


78  INTERCHANGEABLE   MANUFACTURING 


COMPONENT   DRAWINGS  79 

Functional  Requirements  of  the  Mechanism.  The  following 
functional  conditions  must  be  maintained:  The  primer  must  be 
seated  in  the  spindle  plug  in  such  a  manner  that  no  gases  can 
escape  when  the  gun  is  fired.  Any  leakage  of  these  gases,  which 
are  at  a  very  high  temperature  and  under  high  pressure,  will 
quickly  erode  or  burn  out  the  parts  of  the  mechanism,  thus 
destroying  its  effectiveness.  This  requires  that  the  surfaces  of 
the  seat  for  the  primer  be  smooth  and  that  its  dimensions  be 
maintained  within  close  limits.  The  blow  imparted  by  the 
striker  must  be  sufficient  to  insure  that  the  primer  will  always 
be  detonated,  since  the  sole  object  of  the  mechanism  is  to  de- 
tonate the  primer.  In  order  to  insure  this  result,  the  firing  pin 
must  always  protrude,  in  operation,  a  certain  minimum  distance 
(determined  by  experiments),  while,  in  order  not  to  pierce  the 
primer  cup,  it  must  never  protrude  beyond  a  certain  maximum 
distance  (also  determined  by  extensive  experiments).  The  vari- 
ous unit  assemblies  of  the  mechanism  must  be  interchangeable 
in  order  to  allow  quick  replacements  in  service  —  a  vital  re- 
quirement. As  far  as  proves  economical,  the  various  component 
parts  of  the  unit  assemblies  should  be  interchangeable  to  permit 
ready  repairs  in  service.  Unless  noted  otherwise,  the  parts  of 
this  mechanism  must  be  interchangeable.  These  are  the  most 
important  of  the  functional  requirements.  Others  will  be  dis- 
cussed as  they  arise  in  connection  with  the  details. 

Drawing  of  Firing  Pin  Guide.  A  detail  drawing  of  the  firing 
pin  guide  is  shown  in  Fig.  2.  The  outside  diameter  is  0.782  inch 
plus  o.ooo,  minus  0.003.  This  guide  must  be  an  easy  slide  fit  in 
the  container.  It  has  a  minimum  clearance  of  0.002  inch,  as 
will  be  seen  by  a  comparison  with  that  part  of  the  container 
(see  Fig.  7)  which  receives  the  guide.  With  a  tolerance  of  0.003 
inch  on  each  part,  it  has  a  maximum  clearance  of  0.008  inch. 
With  a  reasonably  smooth  finish,  such  as  that  obtained  by  a 
finishing  cut  on  the  guide  and  a  finish-reaming  operation  on  the 
hole  in  the  container,  these  clearances  will  maintain  the  condi- 
tions required. 

The  firing  pin  must  be  an  easy  slide  fit  in  the  guide.  The 
diameter  of  the  firing  pin  hole  is  0.118  inch  plus  0.003,  minus 


8o 


INTERCHANGEABLE   MANUFACTURING 


o.ooo.  The  diameter  of  the  firing  pin  is  0.117  inch  plus  o.ooo, 
minus  0.003;  hence  the  minimum  clearance  is  o.ooi  and  the 
maximum  clearance  (with  tolerance  of  0.003  on  each  part),  0.007 
inch.  With  a  reamed  finish  in  the  hole  and  a  finishing  cut  on 
the  pin,  these  clearances  will  maintain  the  proper  conditions. 

The  diameter  of  the  large  counterbore  in  the  rear  of  the  guide 
is  0.584  inch  plus  0.003,  minus  o.ooo.  The  flange  of  the  firing 
pin  must  be  an  easy  slide  fit  in  this  counterbore.  The  diameter 
of  the  flange  is  0.582  inch  plus  o.ooo,  minus  0.003;  therefore  the 
minimum  clearance  is  0.002  and  the  maximum  clearance,  0.008 


0.118+8:8$ 


-\l 

k^ 


Machinery 


Fig.  2.     Firing  Pin  Guide 

inch.  With  a  reamed  finish  in  the  counterbore  and  a  finishing 
cut  on  the  flange  of  the  firing  pin,  these  clearances  will  maintain 
the  proper  conditions. 

The  diameter  of  the  smaller  counterbore  is  0.484  plus  o.oi, 
minus  o.oo.  This  counterbore  contains  the  firing  pin  guide 
spring.  The  minimum  clearance  is  0.020  and  the  maximum 
clearance  0.040  inch.  It  is  apparent  that  this  surface  is  of  minor 
importance.  The  only  limit  to  the  increase  in  diameter  of  this 
surface  is  controlled  by  the  width  of  shoulder  at  its  mouth  which 
is  needed  to  act  as  a  stop  for  the  firing  pin.  The  basic  width  of 
this  shoulder  is  0.05  inch;  the  tolerance  on  the  counterbore 
diameter  is  o.oi,  thus  reducing  the  effective  width  of  the  shoulder 
by  0.005  inch.  The  tolerance  specified  should  be  sufficient  to 
enable  this  counterbore  to  be  finished  in  one  cut.  No  finishing 


COMPONENT   DRAWINGS  8 1 

operations  are  necessary  on  this  surface  either  for  smoothness 
or  accuracy. 

Exception  to  General  Rule  for  Basic  Dimensions.  The 
length  of  the  guide  (Fig.  2)  is  0.525  inch  plus  0.003,  minus  o.ooo. 
This  dimension  is  an  exception  to  the  general  rule  of  making  the 
basic  dimension  represent  the  maximum  metal  conditions,  be- 
cause of  the  functional  conditions  which  must  be  maintained  in 
this  case.  When  the  firing  pin  and  the  guide  are  seated  solidly 
in  the  container,  the  face  of  the  guide  and  the  end  of  the  firing 
pin  should  be  as  nearly  flush  as  possible.  Under  no  conditions 
must  the  firing  pin  project,  because  any  such  projection  makes 
possible  a  premature  explosion  of  the  primer.  The  basic  dimen- 
sions on  the  firing  pin  and  guide  are  identical  for  this  point,  thus 
making  these  surfaces  flush  under  basic  conditions.  The  direc- 
tion of  the  tolerances  in  each  case  is  such  that  the  pin  can  never 
project.  This  method  of  dimensioning,  therefore,  adheres  to 
the  principle  of  making  the  basic  dimensions  represent  the  danger 
point,  while  the  direction  of  the  tolerance  is  such  as  to  move 
away  from  this  danger  point. 

On  the  other  hand,  there  is  another  danger  point  in  the  other 
direction,  although  not  as  serious  a  one  as  the  first.  In  such 
cases,  the  basic  dimension  should  always  represent  the  more 
dangerous  point,  while  the  tolerances  should  limit  the  extent  of 
the  other.  In  this  case,  the  second  danger  point  is  that  the  end 
of  the  firing  pin  should  be  held  as  nearly  flush  with  the  face  of  the 
guide  as  possible  so  as  not  to  form  a  pocket  into  which  the  primer 
cup  might  be  forced  under  firing  conditions.  If  this  happens,  it 
is  very  difficult  to  remove  the  exploded  primer.  This  may  retard 
the  rate  of  fire  and  possibly  put  the  gun  out  of  action.  The 
tolerances  on  these  dimensions  limit  the  depth  of  this  pocket  to 
0.006  inch  which  is  as  great  as  is  considered  safe.  The  front 
face  of  the  guide  must  be  smooth;  a  polished  surface  is  desir- 
able, as  this  facilitates  the  insertion  and  removal  of  the  primers. 

The  dimension  from  the  bottom  of  the  large  counterbore  to 
the  front  face  is  0.395  inch  plus  o.ooo,  minus  0.004.  This  di- 
mension controls  the  protrusion  of  the  firing  pin  and  is,  there- 
fore, the  dimension  to  be  maintained.  Experiments  show  that 


82  INTERCHANGEABLE   MANUFACTURING 

the  firing  pin  should  protrude  at  least  0.026  inch  in  order  to 
insure  detonation,  while  it  should  not  protrude  over  0.034  inch 
or  there  will  be  danger  of  piercing  the  primer  cup;  therefore, 
the  corresponding  length  of  the  firing  pin  is  made  0.421  inch 
.which  gives  the  minimum  protrusion  of  0.026  inch  while  the 
tolerance  of  0.004  inch  applied  to  each  part  limits  the  maximum 
protrusion  to  0.034  inch. 

Maintaining  a  Common  Locating  Point.  Inasmuch  as  the  im- 
portant functional  dimensions  of  length  are  given  from  the  front 
face,  the  bottom  of  the  small  counterbore  is  also  located  from 
that  surface  so  as  to  maintain  one  common  locating  point.  This 
surface  is  unimportant;  a  dimension  of  0.136  inch  plus  o.oo, 
minus  o.oi  is  specified,  which  should  give  wide  enough  limits  to 
enable  it  to  be  machined  in  a  single  cut. 

The  bevel  at  the  front  of  the  guide  is  provided  to  assist  in 
the  insertion  of  the  primer.  The  diameter  of  the  intersection 
of  this  bevel  with  the  front  face  is  given  as  0.551  inch  plus  o.oo, 
minus  0.02.  These  limits  should  be  wide  enough  to  meet  any 
normal  manufacturing  conditions.  The  surface  of  the  bevel 
must  be  smooth;  a  polished  surface  would  be  desirable.  The 
angle  of  the  bevel  is  given  as  25  degrees.  No  tolerance  is  specified, 
as  the  permissible  variation  is  controlled  by  the  tolerance  given 
for  the  face.  The  angle  of  the  corresponding  surface  of  the 
primer  extractor  is  14  degrees.  The  angle  on  the  guide  is  made 
greater  to  insure  that  the  forward  corner  of  the  guide  will  not 
project  above  the  bottom  of  the  primer  slot  in  the  extractor. 
Any  such  projection  would  interfere  with  the  ready  insertion  of 
primers. 

No  tolerances  are  given  for  the  radii  of  the  corners  of  the 
guide.  In  the  first  place,  a  reasonable  variation  is  already  estab- 
lished for  them  by  the  tolerances  given  on  other  dimensions. 
In  the  second  place,  their  exact  contour  is  of  no  importance, 
their  purpose  being  to  remove  the  sharp  corners.  A  straight 
bevel  of  the  same  dimensions  would  be  as  effective. 

Drawing  of  Firing  Mechanism  Container  Cover.  The  thread 
of  the  container  cover  screws  into  the  container  and  must  be 
set  up  as  tightly  as  possible.  The  outside  or  major  diameter 


COMPONENT   DRAWINGS 


of  the  thread  is  1.125  mcn  plus  o.ooo,  minus  0.008  (see  Fig.  3). 
The  pitch  diameter  is  1.0979  inch  plus  o.ooo,  minus  0.004.  The 
minimum  clearance  is  o.ooo  and  the  maximum  clearance  on 
the  pitch  diameter,  0.008  inch,  as  will  be  seen  by  comparing 
Figs.  3  and  7.  This  tolerance  should  be  kept  as  small  as  normal 
manufacturing  methods  will  permit. 

The  diameter  of  the  flange  is  1.25  inch  plus  o.oo,  minus  o.oi. 
This  surface  is  an  atmospheric  fit  and  of  little  importance,  as 
regards  either  smoothness  or  accuracy;  hence  it  should  be  com- 
pleted in  a  single  machining  operation.  The  diameter  of  the 
stem  is  i.io  inch  plus  o.oo,  minus  o.oi.  This  surface  is  an 


0  «o'+0-00t> 

w.o^u  _  „  uoo 


24-Thds.  per  in.- U.S.  Form-R.H. 
Pitch  Dia.  1.0979  ±  $$g('t  / 

Core  Dia.  1.071 1£°°0" 
Dia.  of  Undercut. 


^i.io'ia'— H 


0.494  i^" 

I    ft  cv+°-°°" 
0.51_o.oi" 

}* 1.544'i0-00-" 


Machinery 


Fig.  3.     Firing  Mechanism  Container  Cover 

atmospheric  fit  and  should  be  machined  in  a  single  operation. 
The  distance  across  the  flats  on  the  stem  is  0.945  inch  plus  o.oo, 
minus  0.02.  This  surface  is  for  the  wrench  used  in  assembling 
and  is  of  little  importance.  It  should  be  machined  in  a  single 
operation  by  a  straddle-milling  tool. 

The  diameter  of  the  hole  and  the  width  of  slot  is  0.520  inch 
plus  0.006,  minus  o.ooo.  These  surfaces  are  for  the  striker  and 
lanyard  connection.  They  should  be  as  smooth  as  careful  ream- 
ing and  finish-milling  operations  will  leave  them.  The  hole  and 
the  slot  must  be  matched  so  that  no  shoulder  will  be  left  at  their 
intersection,  which  would  retard  the  striker  in  its  action.  This 


INTERCHANGEABLE   MANUFACTURING 


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COMPONENT   DRAWINGS  85 

ing  point.  As  the  front  surface  is  the  logical  working  point, 
this  has  been  chosen. 

The  length  of  the  thread  is  0.394  inch  plus  o.oo,  minus  o.oi. 
The  width  of  the  recess  is  0.08  inch  plus  o.oi,  minus  o.oo.  The 
requirements  of  these  dimensions  are  that  there  shall  be  suffi- 
cient threads  to  hold  the  cover  in  position  and  that  the  length 
of  this  stem  shall  be  less  than  the  depth  of  the  recess  in  the  con- 
tainer, as  the  cover  must  always  seat  on  its  flange.  The  depth 
of  the  recess  into  which  the  thread  projects  may  extend  about 
o.oi  inch  below  the  bottom  of  the  threads  to  give  a  suitable 
clearance  for  threading.  Also,  the  end  of  the  stem  and  edge  of 
the  recess  may  be  beveled  about  thirty  degrees  to  facilitate  this 
operation.  The  depth  of  the  counterbore  is  identical  with  the 
length  of  the  threaded  stem.  This  depth  is  of  relatively  small 
importance. 

The  location  of  the  rear  face  of  the  flange  is  0.494  plus  o.oo, 
minus  o.oi.  This  surface  is  an  atmospheric  fit  and  should  be 
finished  in  a  single  operation.  The  bottom  of  the  wrench  cuts 
is  0.51  plus  o.oo,  minus  o.oi.  This  is  a  clearance  surface  of  little 
importance.  It  is  left  above  the  rear  surface  of  the  flange  to 
eliminate  all  matching  operations.  The  bottom  of  the  slot  is 
located  by  these  same  dimensions,  and  its  surface  is  of  equal 
unimportance. 

The  length  over  all  -is  1.544  inches  plus  o.oo,  minus  0.02. 
The  rear  surface  of  the  cover  is  an  atmospheric  fit  of  minor  im- 
portance. No  tolerances  have  been  specified  for  any  of  the 
radii,  as  their  exact  contour  is  of  no  importance.  Their  purpose 
is  to  remove  the  sharp  corners.  Furthermore,  sufficient  varia- 
tions have  been  established  for  these  radii  by  the  tolerances  on 
other  dimensions. 

Drawing  of  Striker.  The  diameter  of  the  front  end  of  the 
striker  (see  Fig.  4)  is  0.591  inch  plus  o.oo,  minus  o.oi.  This 
surface  must  clear  the  counterbore  in  the  container  (see  Fig.  7). 
The  minimum  clearance  is  0.019  inch  and  the  maximum  clear- 
ance 0.039  inch.  The  radius  of  the  air  grooves  is  0.07  inch  plus 
o.oi,  minus  o.oo.  No  finishing  cuts  are  required  on  these  sur- 
faces. The  stem  which  has  a  diameter  of  0.38  inch  plus  o.ooo, 


86 


INTERCHANGEABLE   MANUFACTURING 


minus  0.005,  must  be  a  free  sliding  fit  in  the  spring  seat  washers. 
The  minimum  clearances  are  0.002  inch  and  the  maximum  0.012 
inch.  This  stem  must  have  a  smooth  surface,  such  as  will  be 
secured  by  a  finishing  tool. 

The  rear  shoulder  which  has  a  diameter  of  0.517  inch  plus 
o.ooo,  minus  0.005,  must  be  a  free  sliding  fit  in  the  cover.  The 
minimum  clearance  is  0.003  mcn  an<^  tne  maximum  clearance 
0.014  inch.  This  surface  must  be  as  smooth  as  a  careful  finish- 
ing cut  will  leave  it.  The  length  of  the  front  end  (0.829  inch 
plus  o.ooo,  minus  0.005)  should  be  held  within  reasonably  close 


SOLID  HEIGHT,   NOT  MORE  THAN   0.621" 

ASSEMBLED  HEIGHT.  1.4&" 

LOAD  AT  ASSEMBLED  HEIdHT.NOT  LESS  THAN 

3.  3  LBS. 

SPRING  TO  BE  COMPRESSED  TO  A  HEIGHT  0.77l" 

FOR  48  HOURS,  AFTER  WHICH  IT  SHALL  SUPPORT 

A  LOAD  OF  27.7  LBS.   ATA  HEIGHT  OF  0.77l'±0.03 


FREE  HEIGHT       "''  V_  0.092" 

1.641'—          —  > 


MarMncry 


Fig.  5.     Spring  Seat  Washer 


Fig.  6.    Firing  Spring 


limits  in  'order  to  insure  a  uniform  blow  on  the  firing  pin.  Varia- 
tions in  this  dimension  will  affect  the  force  of  this  blow.  The 
front  face  of  the  striker  must  be  as  smooth  as  a  finishing  cut 
will  leave  it.  The  length  of  the  stem  (1.70  inches  plus  o.oi, 
minus  o.oo)  should  also  be  held  within  reasonably  close  limits, 
as  it  controls,  to  a  certain  extent,  the  force  of  the  blow  of  the 
striker.  This  stem  could  be  machined  with  a  form  tool,  the 
roughing  tool  being  made  to  form  the  neck  for  assembling  the 
washers.  The  neck  is,  therefore,  located  from  the  front  end  of 
the  stem,  the  dimension  being  0.23  inch  plus  o.oi,  minus  o.oi. 
The  width  is  0.145  mc^  P^us  °-OI>  minus  o.oo.  These  limits 
should  be  adequate  to  permit  this  groove  to  be  finished  with  a 
roughing  tool  without  any  unnecessary  refinements. 


COMPONENT   DRAWINGS  87 

The  location  of  the  rear  shoulder  from  the  front  end  (2.73 
inches  plus  o.oo,  minus  0.02)  and  the  width  of  the  bottom  of 
the  groove  (0.40  inch  plus  o.oi,  minus  o.oo)  are  relatively  unim- 
portant. The  surfaces,  however,  must  be  reasonably  smooth 
ones  such  as  are  obtained  with  a  finishing  tool.  These  limits 
should  be  sufficient  for  all  manufacturing  purposes.  The  length 
over  all  (3.30  inches  plus  o.oo,  minus  0.02)  is  also  relatively 
unimportant.  A  sufficiently  smooth  surface  for  the  rear  end 
will  be  obtained  with  a  cutting-off  tool. 

No  tolerances  are  given  for  the  45-degree  bevel  at  the  rear 
end,  nor  for  the  radii,  because  none  are  required.  A  reasonable 
variation  is  already  established  by  tolerances  given  on  other 
dimensions.  Sufficiently  accurate  radii  will  be  obtained  by 
touching  with  a  file  the  various  corners  which  are  not  broken  by 
form  tools,  to  remove  the  sharp  edges. 

Drawing  of  Spring  Seat  Washer.  If  a  large  number  of  spring 
seat  washers  (see  Fig.  5)  were  to  be  manufactured,  they  might 
be  made  in  a  punch  and  die.  The  surface  obtained  in  a  well 
made  sub-press  die  would  be  sufficiently  smooth,  but  the  surface 
obtained  on  the  usual  punch  press  in  an  open  die  would  prob- 
ably require  some  polishing.  For  a  small  number  of  parts,  bar 
stock  could  be  used.  The  surface  obtained  with  a  finishing  tool 
would  be  satisfactory. 

The  hole  must  be  a  free  sliding  fit  on  the  stem  of  the  striker. 
The  minimum  clearance  is  0.002  inch  and  the  maximum  clear- 
ance o.oi  2  inch.  A  surface  equal  to  that  obtained  with  a  reamer 
should  be  secured.  The  width  of  the  assembling  slot  is  unim- 
portant. The  faces  of  the  washer  are  of  but  minor  importance. 
The  original  surface  of  flat  stock,  if  that  is  used,  or  the  surface 
obtained  with  a  cutting-off  tool,  if  bar  stock  is  used,  will  be 
satisfactory. 

Tolerances  are  not  needed  for  the  radius  of  the  corners,  but 
enough  of  the  corner  must  be  removed  to  permit  the  washer  to 
seat  properly  in  the  counterbore  in  the  cover.  This  corner  may 
be  removed  on  a  polishing  wheel  or  with  a  file  in  the  lathe. 

Drawing  of  Firing  Spring.  No  tolerances  are  given  on  the 
dimensions  of  the  firing  spring  (see  Fig.  6),  because  the  functional 


88 


INTERCHANGEABLE   MANUFACTURING 


COMPONENT   DRAWINGS  89 

requirements  are  covered  by  the  weight  specifications,  and  the 
manufacturer  is  allowed  reasonable  latitude  in  these  dimensions 
as  long  as  the  weight  requirements  are  maintained.  The  di- 
mensions given  are  nominal.  A  variation  of  0.005  mcn  m  the 
diameter  of  the  wire,  or  of  0.030  inch  in  the  diameter  of  the  coils 
or  free  length  of  the  spring  will  be  of  no  moment.  If  these  toler- 
ances were  expressed  on  the  drawing,  some  manufacturer  would 
complain  that  the  weight  specifications  would  not  allow  him 
to  take  full  advantage  of  them,  and  would  seek  to  have  the 
weight  requirements  altered  or  removed.  These  weight  con- 
ditions are  the  essential  ones,  as  they  control  the  force  of  the 
blow  on  the  primer.  A  minimum  load  of  3.3  pounds  is  required 
at  the  assembled  height  of  1.45  inches.  A  load  of  27.7  pounds  is 
required  at  a  height  of  0.771  inch  plus  0.03,  minus  0.03.  By 
thus  specifying  loads  at  two  heights,  the  strength  of  the  spring 
is  very  closely  controlled. 

Drawing  of  Firing  Mechanism  Container.  The  container  is 
shown  in  Fig.  7.  The  housing  thread  (which  has  an  outside 
diameter  of  1.417  inches  plus  o.ooo,  minus  0.006)  is  a  special 
thread  and  will  undoubtedly  be  milled.  It  must  be  a  very  free 
fit  in  the  housing,  as  the  container  is  inserted  and  removed 
every  time  the  gun  is  fired.  It  must  assemble  readily,  even  if  a 
certain  amount  of  dirt  and  grit  is  present.  The  minimum  clear- 
ance is  0.008  inch  and  the  maximum  clearance  0.020  inch.  The 
surfaces  must  be  smooth.  A  finish- turning  or  milling  cut  will 
be  satisfactory.  It  will  be  necessary  to  match  the  turning  and 
milling  cuts  where  the  bottom  of  this  thread  matches  the  cylin- 
drical portion  of  the  container  with  a  file  after  the  part  is 
machined. 

The  thread  for  the  primer  extractor  (outside  or  major  diameter 
1.024  inches  plus  o.ooo,  minus  0.008)  must  be  left  hand  to  pre- 
vent the  primer  extractor  from  unscrewing  as  the  mechanism  is 
removed  from  the  housing.  The  minimum  clearance  is  o.ooo 
and  the  maximum  clearance  on  the  pitch  or  effective  diameter, 
0.008  inch.  The  primer  extractor  must  be  screwed  home  as 
firmly  as  possible,  and  the  variations  on  these  threads  should 
be  kept  as  small  as  normal  manufacturing  methods  will  permit. 


go  INTERCHANGEABLE   MANUFACTURING 

The  small  counterbore  in  the  rear  end  (diameter  0.6 10  inch 
plus  o.oi,  minus  o.oo)  is  for  clearance  and  is  unimportant.  It 
should  be  finished  at  a  single  operation  of  a  counterbore.  The 
large  counterbore  in  the  rear  end  (diameter  0.950  inch  plus  0.005, 
minus  o.ooo)  must  be  a  free  sliding  fit  for  the  washer  and  should 
be  reamed.  The  minimum  clearance  between  the  hole  having 
a  diameter  of  0.35  inch  plus  o.oi,  minus  o.oo,  and  the  firing  pin 
is  0.05  inch,  while  the  maximum  clearance  is  0.07  inch.  This 
surface  is  unimportant  and  should  be  machined  by  a  single 
drilling  operation.  The  diameter  of  the  counterbore  in  the  front 
end  is  0.784  inch  plus  0.003,  minus  o.ooo.  This  surface  must  be 
an  easy  sliding  fit  for  the  guide  and  requires  a  careful  finish- 
reaming  operation.  The  length  over  all  (3.271  inches  plus  o.oo, 
minus  0.02)  is  relatively  unimportant,  yet  both  the  front  and 
the  rear  faces  must  be  smooth,  as  they  form  the  seats  for  the 
cover  and  primer  extractor.  The  majority  of  the  length  di- 
mensions are  located  from  the  front  face.  A  few  are  given  from 
the  rear  end  because  of  manufacturing  considerations.  The 
remainder  are  given  from  intermediate  points  because  of  the 
functional  requirements  of  the  mechanism. 

The  length  of  the  thread  for  the  extractor  is  0.209  mch  plus 
o.oi,  minus  o.oo,  and  the  width  of  the  under-cut,  0.06  inch  plus 
0.005,  minus  o.ooo.  The  requirements  of  these  dimensions  are 
that  there  shall  be  sufficient  threads  to  hold  the  extractor  prop- 
erly and  that  the  length  of  the  stem  be  always  long  enough  to 
permit  the  extractor  to  seat  on  the  front  face  of  the  container. 

The  bottom  of  the  large  counterbore,  which  is  i.n  inches 
plus  o.ooo,  minus  0.005,  from  the  firing  pin  seat,  is  an  impor- 
tant functional  surface  and  should  be  finished  by  a  special  opera- 
tion, locating  from  the  firing  pin  seat.  This  dimension  controls, 
to  a  great  extent,  the  force  of  the  blow  of  the  striker.  The 
location  of  the  housing  thread  is  also  controlled  from  the  firing 
pin  seat.  This  dimension  (0.291  inch  plus  o.ooo,  minus  0.005) 
controls  the  angular  position  of  the  mechanism  when  it  is 
screwed  home  in  the  housing.  A  plus  variation  on  this  dimen- 
sion might  prevent  the  mechanism  from  locking.  The  width  of 
the  housing  thread  is  0.405  inch  plus  o.oo,  minus  o.oi.  The 


COMPONENT   DRAWINGS 


rear  surface  or  flank  of  this  thread  is  a  bearing  surface  and  must 
be  smooth.  A  corner  of  this  rear  flank  is  beveled  with  a  file  to 
facilitate  entering  the  housing  (see  view  showing  development  of 
thread).  The  length  of  bevel  is  0.158  inch  plus  0.02,  minus  o.oo, 
and  the  width  of  bevel  0.04  inch  plus  0.02,  minus  o.oo.  This 
should  give  sufficiently  liberal  tolerances  for  all  manufacturing 
purposes.  Double  this  tolerance  could  be  given,  however,  if 
necessary. 

The  latch-pin  and  spring  holes  are  both  located  from  the 
front  face  of  the  container,  as  this  is  the  logical  locating  point 
for  the  drill  jig.  The  distance  from  the  latch-pin  hole  to  the 


24  Thds.U.S.F.  L.H. 
Pitch  Dia.0.9969  ±00;0°S 
Core  Dia.  0.9698'+  o.oos  ^ 

Dia.  of  Undercut+o.ois" 

__  ___  1.024^000^1 

A  —  A 

^- 


n       "+ 0.004" 

°-4<2-o.ooo" 


2  7  «?     0.02V 

_L_l-±; 


0.06  ±8:151!};;  --   - 

o^s?;;  —  Jr 


Machinery 


Fig.  8.     Primer  Extractor 

center  of  the  container  is  0.625  inch  plus  0.005,  minus  o.ooo. 
A  minus  variation  on  this  last  dimension  would  develop  inter- 
ference between  the  bottom  of  the  slot  and  the  latch.  The 
latch-pin  thread  is  a  standard  No.  5-44  A.S.M.E.  thread.  Taps 
should  be  avalaible  from  stock  from  any  reliable  tap  manu- 
facturer. This  is  a  fine  thread  and  should  be  held  as  close  to 
size  as  normal  manufacturing  conditions  will  permit. 

The  distance  to  the  cut  on  the  left  side  of  the  flange  is  0.30 
inch  plus  o.oo,  minus  o.oi,  and  the  height  from  the  bottom  of 
the  cut  is  0.56  inch  plus  o.oo,  minus  o.oi.  The  requirements  of 
this  cut  are  that  the  cutter  shall  not  gouge  the  knurled  handle 


§2  INTERCHANGEABLE   MANUFACTURING 

by  cutting  too  deeply  and  that  the  tap  shall  not  gouge  the 
bottom  of  this  cut.  The  limits  specified  give  the  greatest  per- 
missible variations  under  maximum  metal  conditions  and  should 
be  great  enough  to  allow  this  cut  to  be  machined  in  a  single 
operation.  The  width  of  the  latch  slot  is  0.300  inch,  plus  0.006, 
minus  o.ooo.  This  surface  must  be  reasonably  smooth  so  as  to 
maintain  an  easy  action  of  the  latch.  The  minimum  clearance 
is  0.003  inch  and  the  maximum  clearance,  0.013  inch.  With 
the  proper  surfaces,  these  clearances  will  maintain  the  desired 
conditions. 

The  location  of  the  bottom  of  the  slot  from  the  center  of  the 
container  is  0.45  inch  plus  o.oo,  minus  0.02.  A  plus  variation 
on  this  dimension  would  develop  interference  with  the  latch. 
The  surface  of  the  bottom  and  ends  of  this  slot  is  not  important 
and  requires  no  finishing  cuts.  The  cuts  on  the  side  of  the  slot 
must  be  matched  to  provide  a  smooth  bearing  for  the  latch. 

The  ends  of  the  housing  thread  are  0.02  inch  plus  0.02,  minus 
o.oo,  from  the  center  of  the  container.  These  surfaces  require 
no  finishing  cuts.  The  length  of  the  knurling  is  1.60  inches 
plus  o.oo,  minus  0.05.  A  scale  measurement  will  be  sufficient 
to  check  this  dimension.  No  tolerances  are  given  on  the  vari- 
ous radii  or  angles,  as  none  are  required.  Tolerances  on  other 
dimensions  establish  liberal  variations  for  these  surfaces. 

Drawing  of  Primer  Extractor.  The  outside  diameter  of  the 
primer  extractor  (see  Fig.  8)  which  is  1.102  inches,  plus  o.ooo, 
minus  0.006,  should  approximately  match  the  corresponding 
diameter  on  the  container  shown  in  Fig.  7.  The  surface  should 
be  reasonably  smooth.  The  diameter  of  the  recess  for  the  firing 
pin  guide  (0.791  inch,  plus  o.oi,  minus  o.oo)  is  clearance  and  is 
unimportant.  The  depth  (0.264  inch,  plus  0.005,  minus  o.ooo) 
is  more  important,  as  it  controls  the  amount  of  surface  which 
engages  the  head  of  the  primer.  A  finish  cut  will  be  required 
on  this  surface. 

The  width  of  the  extractor  slot  (0.472  inch,  plus  0.004,  minus 
o.ooo)  is  an  important  dimension  and  the  surface  must  be 
smooth.  A  finish  cut  will  be  required.  The  bevel  at  the  end 
of  this  slot  is  at  an  angle  of  30  degrees  and  is  located  from  the 


COMPONENT  DRAWINGS  93 

center  of  the  extractor  at  a  distance  of  0.236  inch,  plus  o.oi, 
minus  o.oi.  The  exact  dimensions  of  the  bevel  are  unimpor- 
tant, as  it  is  provided  merely  to  facilitate  assembling  the  primer. 
The  surfaces  must  be  smooth,  however,  even  if  an  extra  filing 
operation  is  needed  to  match  the  cuts. 

The  distance  across  the  flats  (0.945  inch,  plus  o.oo,  minus 
o.oi)  is  for  the  wrench  used  in  assembling  and  is  unimportant. 
No  finish  cuts  are  required.  The  length  of  the  extractor  is 
0.394  inch,  plus  o.oo,  minus  o.oi.  The  front  face  must  clear  the 
rear  face  of  the  spindle  plug  when  the  primer  is  seated;  there- 
fore, no  plus  variation  is  permissible.  Any  great  minus  varia- 
tion will  weaken  the  extractor.  The  tolerance  given  should  be 
liberal  enough  for  all  normal  manufacturing  purposes.  Both 
the  front  and  rear  surfaces  should  be  reasonably  smooth.  This 
will  require  finishing  cuts. 

The  depth  of  the  tapped  hole  is  0.209  inch,  plus  o.oo,  minus 
o.oi,  and  the  width  of  the  thread  under-cut,  0.06  inch,  plus 
0.005,  minus  o.ooo.  Enough  threads  must  be  secured  to  hold 
the  extractor  firmly  in  position,  yet  the  depth  must  be  shallow 
enough  to  permit  the  extractor  to  seat  on  the  front  face  of  the 
container.  It  is  permissible  to  make  the  depth  of  the  thread 
under-cut  not  over  0.005  inch  below  the  bottom  of  the  threads 
to  provide  clearance  for  the  tap.  A  greater  diameter  of  under- 
cut than  1.042  inch  (maximum  outside  or  major  diameter  of 
thread  or  1.032  inch  plus  o.oi  in  diameter  allowed  for  tap 
clearance)  would  weaken  the  extractor  to  such  an  extent  that 
it  would  not  be  safe  to  use  it  in  service. 

The  location  of  the  bottom  of  the  primer  slot  from  the  rear 
face  is  0.248  inch,  plus  o.ooo,  minus  0.005.  This  surface  should 
never  come  below  the  corner  on  the  firing  pin  guide,  for  if  it 
did,  it  would  be  difficult  to  insert  the  primer.  The  surface 
should  be  smooth  and  all  corners  about  this  slot  must  be  care- 
fully broken. 

Dimensioning  to  Prevent  Compound  Tolerances.  The  coun- 
tersink which  merges  into  the  beveled  surface  on  the  under  side 
of  the  primer  head  is  located  from  a  theoretical  point,  where  its 
angle  of  35  degrees  intersects  the  center  line  of  the  extractor. 


94 


INTERCHANGEABLE    MANUFACTURING 


The  distance  from  this  intersecting  point  to  the  front  face  is 
0.306  inch,  plus  0.005,  minus  o.ooo.  Such  a  method  of  dimen- 
sioning is  necessary  to  prevent  compound  tolerances.  It  will 
be  noted  that  no  dimensions  are  given  for  the  intersections  of 
this  angle  with  the  primer  slot  or  bottom  of  the  recess.  Such 
dimensions  are  unnecessary  and  could  not  be  measured  directly 
in  any  event.  The  dimensions  given  locate  this  surface  defi- 
nitely and  completely.  It  will  be  necessary  for  the  manufacturer 
to  compute  the  diameters  on  this  countersink  to  suit  his  own 


ENDS  GROUND  SQUARE 


6  COILS  NO.  10  MUSIC  WIRE 


No.5  44-U.S.Form-R.H. 
Pitch  Dia.  0.1102  +£$&'' 
Core  Dia.  0.0955 +0.000;; 


Machinery 


Fig.  9.    Locking  Latch,  Spring  and  Pin 

particular  needs.  This  surface  must  be  smooth  and  will  require 
a  careful  finishing  operation. 

No  tolerances  are  given  on  any  of  the  angles  or  radii  because 
none  are  needed.  Sufficient  variation  on  these  surfaces  is  per- 
mitted by  the  tolerances  given  on  other  dimensions. 

Drawing  of  Locking  Latch,  Spring,  and  Pin.  The  surfaces  of 
the  locking  latch  (Fig.  9)  must  be  smooth,  as  they  bear  on  the 
sides  of  the  slot  in  the  container.  This  part  has  a  tolerance  of 
minus  o.oi  inch  on  the  entire  contour.  This  means  that  the 


COMPONENT  DRAWINGS 


95 


S3  I  §38  33 

e>o   0-d  oo   oo 

+ 1  +1+1  +1 


96  INTERCHANGEABLE   MANUFACTURING 

piece  may  vary  o.oi  inch  normal  to  the  profile  at  any  point  in 
the  direction  that  will  make  the  piece  smaller,  or,  in  other  words, 
any  variation  from  the  normal  dimensions  must  remove  more 
metal.  The  diameter  of  the  pin  hole  (0.092  inch,  plus  0.004, 
minus  o.ooo)  corresponds  with  the  pin  hole  in  the  container. 
The  diameter  of  the  spring  hole  (0.175  inch,  plus  o.oi,  minus 
o.oo)  also  corresponds  with  the  spring  hole  in  the  container. 

The  locking  latch  spring  (Fig.  9)  is  a  part  of  minor  importance. 
It  is  made  of  No.  10  music  wire,  and  no  tolerance  is  specified  for 
its  diameter.  This  means  that  commercial  music  wire  bought 
in  the  open  market  will  be  satisfactory.  No  difficulty  should 
be  experienced  in  maintaining  the  limits  given. 

The  thread  of  the  locking  latch  pin  is  a  No.  5-44  U.  S.  form. 
This  is  a  standard  A.S.M.E.  thread,  and  dies  should  be  avail- 
able in  stock  at  any  reliable  die  manufacturer's.  After  this 
pin  is  assembled  into  the  container,  the  end  thread  of  the  tapped 
hole  in  the  container  should  be  upset  slightly  with  a  punch  to 
prevent  this  pin  from  falling  out.  It  should  be  understood  that 
it  is  permissible  to  bevel  at  both  ends  of  the  thread  to  facilitate 
the  threading.  The  surface  of  the  stem  forms  a  bearing  for  the 
latch  and  should  receive  a  finish  cut.  This  part  should  be  com- 
pleted in  a  single  operation  on  a  screw  machine. 

Drawing  of  the  Housing.  The  outside  diameter  of  the  hous- 
ing (Fig.  10)  is  2.638  inches,  plus  o.oo,  minus  o.oi.  This  is  an 
atmospheric  fit  and  requires  no  finishing  cut,  which  also  applies 
to  the  shoulder,  the  diameter  of  which  is  1.925  inches  plus  o.oo, 
minus  o.oi.  The  Whitworth  thread,  the  full  or  major  diameter 
of  which  is  2.100  inches,  plus  0.012,  minus  o.ooo,  must  assemble 
on  the  hinged  collar  (Fig.  n).  The  Whitworth  form  of  thread 
is  used  to  suit  other  types  of  firing  mechanisms  now  in  service; 
otherwise  the  U.  S.  form  of  thread  would  be  preferable. 

The  counterbore  in  the  front  end  (diameter  1.502  inches, 
plus  0.02,  minus  o.oo)  must  clear  the  spindle  plug.  No  finish- 
ing cut  is  required.  The  counterbore  in  the  rear  end  of  the  same 
size  must  clear  the  flange  on  the  container.  The  minimum 
clearance  is  0.007  incri  and  the  maximum  clearance,  0.047  inch. 
No  finish  cut  is  required.  The  full  or  major  diameter  of  the  con- 


COMPONENT  DRAWINGS 


97 


tamer  thread  is  1.425  inches,  plus  0.006,  minus  o.ooo.  A  sector 
of  this  thread  is  removed  to  permit  the  assembly  of  the  con- 
tainer. All  of  these  surfaces  require  finishing  cuts.  The  rear 
flank  is  the  bearing  flank  and  the  most  essential.  The  re- 
quirements were  previously  given  in  connection  with  the  firing 
mechanism  container.  The  front  face  of  the  housing  should  be 
reasonably  smooth,  as  it  seats  against  the  hinged  collar  and  is 
the  most  important  working  point  for  other  machining  operations. 
The  depth  of  the  Whitworth  thread  is  0.80  inch,  plus  0.02, 
minus  o.oo.  The  hole  must  be  deep  enough  for  the  hinged  collar, 


0.160 


Machinery 


Fig.  11.     Hinged  Collar  assembled 

and  enough  threads  must  be  maintained  to  hold  the  firing 
mechanism  in  position  during  the  firing  of  the  gun.  This  thread 
is  subjected  to  a  considerable  strain  at  this  time.  The  depth 
of  the  counterbore  in  the  front  end  is  1.08  inches,  plus  0.02, 
minus  o.oo.  This  surface  must  clear  the  end  of  the  spindle  plug. 
No  finish  cut  is  required. 

The  location  of  the  thread  for  the  container  is  1.987  inches, 
plus  0.005,  minus  o.ooo  from  the  front  end.  This  is  an  impor- 
tant functional  dimension  and  must  be  carefully  watched,  as 
it  controls  the  angular  position  of  the  firing  mechanism  when 


98  INTERCHANGEABLE   MANUFACTURING 

it  is  screwed  home.  A  minus  variation  on  this  dimension  would 
prevent  the  mechanism  from  locking.  A  slot  is  shown  in  the 
lower  right-hand  side  of  the  housing  for  the  safety  mechanism. 
The  safety  bar  should  be  a  very  free  fit  in  this  slot.  The  slot 
is  1.59  inches,  plus  o.oi,  minus  o.oi  from  the  front  end;  the 
length,  0.75  inch,  plus  0.02,  minus  o.oo;  and  the  width,  0.375 
inch,  plus  0.02,  minus  o.oo.  The  surface  in  the  slot  should  be 
reasonably  smooth.  A  tapped  hole,  having  a  full  or  major 
diameter  of  0.50  inch,  plus  o.oi,  minus  o.oo,  is  shown  in  the 
bottom  of  the  housing  for  the  firing  mechanism  pin.  It  is  per- 
missible to  run  the  tap  drill  below  the  thread,  provided  that 
this  drill  does  not  break  through  into  the  hole  in  the  center  of 
the  housing. 

A  recess  is  milled  in  the  rear  face  of  the  housing  to  engage 
the  locking  latch.  This  recess  allows  for  a  possible  variation 
in  the  locked  position  of  the  container  of  90  degrees.  The  toler- 
ances given  on  the  various  controlling  dimensions  will  permit  a 
variation  of  approximately  30  degrees.  The  variations  on  the 
primer  plug  and  the  primer  will  permit  approximately  30  degrees 
more.  This  leaves  the  remaining  30  degrees  to  allow  for  wear. 
The  depth  of  the  recess  is  o.io  inch,  plus  0.02,  minus  o.oo  and 
the  ends  of  the  recess  are  located  0.15  inch,  plus  o.oi,  minus 
o.oo  from  the  center  lines  through  the  rear  face,  90  degrees 
apart. 

The  hole  and  counterbores  for  the  collar-catch  (shown  in 
Fig.  12)  are  located  from  the  center  of  the  housing  because  these 
holes  will  be  drilled  in  a  jig  which  should  locate  the  housing 
centrally.  The  counterbores  are  1.088  inches,  plus  o.ooo,  minus 
0.005,  and  the  hole,  1.183  inches,  plus  0.005,  minus  o.ooo  from 
the  center  line  (see  end  view  Fig.  10).  The  diameter  of  the 
hole  is  o.i 6  inch,  plus  0.006,  minus  o.ooo.  This  hole  should  be 
a  free  sliding  fit  for  the  stem  of  the  collar-catch.  The  mini- 
mum clearance  is  0.002,  and  the  maximum  clearance,  0.014  inch. 
The  full  or  major  diameter  of  the  tapped  hole  is  0.375  inch, 
plus  0.008,  minus  o.ooo,  and  the  pitch  diameter,  0.3417  inch, 
plus  0.004,  minus  o.ooo.  No  core  or  minor  diameter  is  given, 
as  the  small  counterbore,  which  is  a  few  thousandths  inch  larger 


COMPONENT   DRAWINGS 


99 


then  the  theoretical  core  diameter,  limits  the  height  of  the 
threads.  The  V-form  of  thread  is  used  to  obtain  as  great  an 
area  of  contact  as  possible.  After  assembly  of  the  collar-catch 
screw,  the  metal  should  be  upset  slightly  with  a  cold  chisel  into 
the  slot  of  the  screw  to  prevent  disassembling.  The  bottom  of 
the  counterbore  is  0.125  inch,  plus  0.02,  minus  o.oo  from  the 
rear  face  of  the  body.  This  counterbore  receives  the  head  of 
the  collar-catch  screw  and  also  forms  a  groove  in  the  stem 
which  provides  clearance  for  drilling  and  tapping. 

The  width  of  the  thread  for  the  container  is  0.41  inch,  plus 
o.oi,  minus  o.oo.  A  45-degree  bevel  (0.03  inch,  plus  0.02,  minus 
o.oo  wide)  is  required  on  the  corner  of  this  thread  to  facilitate 


TOLERANCE          'n/  AS  SHOWN 
— 


Machinery 


Fig.  12.    Collar-catch 

the  insertion  of  the  container.  This  bevel  may  be  made  with  a 
rile;  the  tolerance  should  be  great  enough  to  cover  this  method 
of  manufacture.  The  ends  of  the  thread  sector  are  located  from 
the  45-degree  center  line  of  the  housing  (see  end  view)  at  0.02 
inch,  plus  0.02,  minus  o.oo.  This  sector  is  at  an  angle  so  as  to 
always  insure  a  minimum  contact  on  this  thread  of  135  degrees. 
No  tolerances  are  given  on  the  various  angles  and  radii,  as  none 
is  required. 

Drawing  of  Hinged  Collar.  The  hinged  collars  and  housings 
are  not  interchangeable  and  must  be  furnished  in  pairs.  To 
make  these  parts  interchangeable  and  insure  that  the  housing 
would  be  screwed  tightly  against  the  shoulder  on  the  hinged 
collar  when  the  locking  holes  in  each  part  were  in  correct  align- 


IOO  INTERCHANGEABLE   MANUFACTURING 

ment,  would  require  very  expensive  manufacturing  methods. 
In  such  a  case,  the  position  of  the  start  of  the  Whitworth  thread 
in  the  housing  would  have  to  be  held  very  closely  in  relation  to 
the  position  of  the  locking  hole.  The  same  would  be  true  on 
the  hinged  collar.  Some  variation  must  of  necessity  be  allowed. 
This  would  introduce  a  further  variation  longitudinally  of  the 
position  of  the  firing  mechanism.  The  effect  of  such  a  variation 
would  be  an  additional  angular  variation  in  the  locked  position 
of  the  mechanism.  If  the  original  pairs  of  housings  and  hinged 
collars  become  separated,  an  additional  locking  hole  will  have 
to  be  drilled  in  the  flange  of  the  collar,  Fig.  n,  transferring  it 
from  the  housing  which  is  to  be  used.  The  diameter  of  the  lock- 
ing hole  is  0.160  inch,  plus  0.006,  minus  o.ooo.  It  should  be 
drilled  in  one  operation  by  using  its  companion  housing  as  a  jig. 

Drawing  of  Collar-catch.  For  convenience  of  manufacture, 
the  collar-catch  (see  Fig.  12)  is  made  in  two  parts  which  are 
permanently  assembled.  The  stem  and  the  finger  piece  may 
be  made  interchangeable,  or  a  system  of  selective  assembly  may 
be  employed.  This  is  matter  to  be  determined  by  the  manu- 
facturer to  suit  his  own  convenience.  Therefore  no  tolerances 
will  be  given  on  the  dimensions  of  the  riveted  end.  The  stem 
must  be  a  snug  fit  in  the  finger  piece,  and  the  two  parts  must  be 
solid  after  riveting.  Any  parts  made  within  o.oi  inch  of  the 
nominal  dimensions  and  which  meet  the  above  conditions  will 
be  acceptable.  The  rear  face  of  the  finger  piece  will  be  finished 
after  riveting. 

As  the  diameter  of  the  stem  is  0.158  inch,  plus  o.ooo,  minus 
0.006,  it  should  be  possible  to  secure  drill  rod  well  within  these 
limits;  no  further  machining  will  be  required  on  this  surface. 
The  length  of  the  stem  is  of  minor  importance.  The  surface  left 
by  the  cutting-off  tool  will  be  satisfactory.  The  length  of  the 
finger  piece  (0.525  inch)  is  an  atmospheric  fit;  no  tolerances  are 
given,  because  the  note  in  regard  to  the  profile  gives  a  tolerance 
of  minus  0.04  inch. 

The  thickness  of  the  flange  (0.125  inch  plus  o.oo,  minus  o.oi) 
is  of  minor  importance;  hence  the  flange  can  be  completed  in 
a  single  operation  after  the  stem  is  riveted.  The  width  of  the 


COMPONENT   DRAWINGS 


101 


flange  (0.245  inch,  plus  o.oo,  minus  o.oi)  must  be  free  in  the 
slot  in  the  housing.  The  minimum  clearance  is  0.005  mcn  and 
the  maximum  clearance,  0.035  inch. 

The  dimensions  of  the  profile  of  the  finger  piece  are  given 
without  tolerances,  but  a  note  is  added:  "Tolerance  plus  o.oo, 
minus  0.02,  as  shown  by  dotted  line."  The  entire  upper  part  of 
the  finger  piece  is  an  atmospheric  fit.  It  must  be  reasonably 
smooth  because  the  finger  operates  this  part.  The  note  permits 
a  minus  variation  of  0.02  on  this  profile  where  no  other  toler- 
ances are  given.  This  variation  is  measured  as  normal  to  the 
profile  at  any  point.  If  a  clean  drop-forging  is  secured,  this 


!« 


a 


>0   +   | 

0.04  R  //  ^-1  5t- 

-0.04   R  .*en     *H 


,77  \ 


1 \_ 

y-T- 

0.30lo.oo;; 


*A      /         ^i§    f\ 

S^  ^r=i:t^-^ 


arhincry 


.frig.  13.     Firing  Pin  Guide  Spring  and  Firing  Pin 

surface  may  be  finished  by  removing  all  rough  scale,  flash,  and 
other  rough  spots  with  a  file  or  on  a  polishing  wheel.  The  con- 
tour of  this  surface  is  not  important  enough  to  require  expen- 
sive form-milling  cuts. 

Drawing  of  Firing-pin  Guide  Spring  and  Firing  Pin.  The 
diameter  of  the  wire  for  the  firing-pin  guide  spring  (Fig.  13)  is 
0.04  inch,  plus  o.ooo,  minus  0.002 ;  the  outside  diameter  of  the 
coils,  0.464  inch,  plus  o.oo,  minus  o.oi;  and  the  free  height,  0.69 
inch,  plus  0.02,  minus  o.oo.  These  limits  should  be  readily 
maintained  under  normal  manufacturing  conditions. 

The  firing-pin  flange  is  0.582  inch  in  diameter,  plus  o.ooo, 
minus  0.003.  This  surface  must  be  a  free  sliding  fit  in  the 


102 


INTERCHANGEABLE    MANUFACTURING 


firing-pin  guide.  The  surface  will  require  a  careful  finishing  cut. 
The  diameter  of  the  front  end  (0.117  inch,  plus  o.ooo,  minus 
0.003)  must  be  a  free  sliding  fit  in  the  firing  pin  guide.  This 
surface  requires  a  careful  finishing  cut. 

The  surface  of  the  rear  end  clears  the  hole  in  the  container 
by  0.050  inch  and  requires  no  finishing  cut.  The  diameter  of 
the  large  end  of  the  taper  is  an  unimportant  clearance  surface. 
The  taper  is  provided  to  strengthen  the  end  of  the  firing  pin. 
No  finishing  cut  is  required  on  this  tapered  surface.  The  length 
over  all  is  an  important  functional  dimension  and,  in  part, 


26  V-Thds.  per  in.  R.H. 
Pitch  Dia.  0.3417'±g;9$>,'/' 
Core  Dia.  0.3804'+ J}-{gjj« 
Dia.of  Undercut  0.3804' t°-9?o: 


3  COILS  NO.  15  MUSIC  WIRE 


MacMnerv 


Fig.  14.     Collar-catch  Screw  and  Spring 

controls  the  force  of  the  blow  on  the  primer.  The  front  face  of 
the  pin  must  be  as  smooth  as  possible,  and  a  polished  surface  is 
desirable.  The  rear  face  should  be  as  smooth  as  a  careful  finish- 
ing cut  will  leave  it. 

The  location  of  the  rear  face  of  the  flange  (0.525  inch,  plus 
o.ooo,  minus  0.003)  *s  important,  as  this  dimension  controls 
the  location  of  the  end  of  the  firing  pin.  The  surface  should 
receive  a  finishing  cut.  The  distance  to  the  front  face  of  the 
flange  is  an  important  functional  dimension  which  controls 
the  protrusion  of  the  firing  pin,  as  noted  in  connection  with  the 
firing-pin  guide.  A  finishing  cut  is  required  on  this  surface.  The 


COMPONENT   DRAWINGS  103 

location  of  the  beginning  of  the  taper  is  relatively  unimportant. 
This  dimension  maintains  clearance  with  the  bottom  of  the 
counterbore  in  the  firing-pin  guide. 

No  tolerances  are  given  for  the  radii  because  none  are  needed. 
Attention  is  called,  however,  to  the  radius  of  0.04  inch  at  the 
front  end.  This  must  not  be  exceeded.  The  purpose  of  this 
radius  is  to  remove  the  sharp  corner,  but  care  must  be  taken  to 
remove  as  little  material  as  possible. 

Drawing  of  Collar-catch  Screw  and  Spring.  The  collar- 
catch  screw  is  shown  in  Fig.  14.  The  diameter  of  the  head  must 
enter  the  counterbore  in  the  housing.  The  thread,  which  is  a 
sharp  V-form,  must  assemble  into  the  tapped  hole  in  the  hous- 
ing. Sufficient  threads  must  be  secured  to  hold  the  screw  in 
position.  It  is  permissible  to  bevel  both  under-cut  and  end  to 
facilitate  threading.  This  screw  should  be  completed  in  a 
single  operation  on  a  screw  machine. 

No.  15  music  wire  is  specified  for  the  collar-catch  spring. 
Commercial  wire  of  this  number  will  be  satisfactory.  The  func- 
tion of  this  spring  is  to  hold  the  collar  catch  in  its  locked  posi- 
tion. The  limits  given  should  be  maintained  readily  under 
normal  manufacturing  conditions. 

All  dimensions  and  tolerances  given  on  these  drawings  repre- 
sent limit  gage  sizes.  If  a  hole  is  given  as  1.25  inches,  plus  o.oi, 
minus  o.oo,  this  means  that  the  hole  must  be  made  so  that  a  plug 
gage  1.25  inches  in  diameter  will  always  enter,  while  a  plug 
gage  1.26  inches  in  diameter  will  not.  In  general,  the  extent  of 
the  tolerances  allowed  on  any  surface  is  a  good  index  of  the  char- 
acter of  the  finish  required.  All  burrs,  fins,  etc.,  and  unneces- 
sary sharp  corners  must  be  removed.  All  cuts  and  surfaces, 
whether  rough  or  finished,  must  show  no  evidence  of  careless- 
ness. All  cuts  must  be  made  with  clean  and  sharp  tools.  Gouges, 
tears,  and  unnecessary  scratches  produced  by  dull  or  improper 
tools  and  careless  workmanship  or  carelsss  handling  should  be 
sufficient  cause  for  rejection. 

Unless  noted  otherwise,  common  manufacturing  practices, 
such  as  under-cutting  and  beveling  for  threads,  extending  the 
tap  drill  a  reasonable  amount  below  the  threads  in  tapped  holes, 


104  INTERCHANGEABLE   MANUFACTURING 

countersinking  to  guide  the  tap,  providing  reasonable  grinding 
clearances  where  necessary,  burr-beveling  corners  on  screw 
machine  parts,  etc.,  are  permissible.  Whenever  any  differences 
exist  between  the  dimensions  and  tolerances  expressed  on  the 
drawing  and  the  above  specifications,  the  figures  on  the  drawing 
should  be  used.  The  dimensions  and  tolerances  given  on  the 
component  drawings  should  be  strictly  maintained.  If  modi- 
fications are  possible  which  will  relieve  the  situation,  they  should 
be  made.  No  deviations  from  the  specified  requirements  are 
permissible,  however,  until  definite  modifications  are  authorized. 


CHAPTER  VII 

ECONOMICAL    PRODUCTION 

WHEN  certain  manufacturing  methods  are  to  be  decided  upon, 
the  decision  made  in  this  connection  should  be  recorded,  to- 
gether with  the  reasons  for  it.  This  practice  tends  to  eliminate 
many  expensive,  unnecessary  refinements  which  are  often  arbi- 
trarily specified,  because,  instead  of  baldly  specifying  the  vari- 
ous requirements,  the  necessity  for  adding  sufficient  reasons 
therefor  demands  a  careful  analysis  of  the  mechanism  and  its 
purpose;  and  a  careful  analysis  of  almost  any  mechanism  will 
soon  make  it  apparent  that  only  a  small  proportion  of  the  di- 
mensions and  other  requirements  are  exacting.  This  and  many 
other  subjects  bearing  upon  the  attainment  of  economical  prac- 
tice in  interchangeable  manufacturing  are  dealt  with  in  the 
present  chapter. 

Principal  Elements  in  Economical  Production.  There  are 
three  principal  elements  in  the  economical  and  successful  pro- 
duction of  a  commodity.  Stated  briefly,  they  are  as  follows: 
(i)  A  thorough  knowledge  of  the  object  (function)  of  the  article 
and  of  all  the  conditions  essential  in  attaining  it.  (2)  The  de- 
velopment of  manufacturing  methods  and  facilities  that  will 
most  economically  produce  a  satisfactory  product.  (3)  The 
development  of  testing  methods  and  apparatus  to  determine  in 
an  economical  manner,  at  any  stage,  whether  or  not  the  desired 
results  are  being  achieved. 

Duplication  of  work  never  results  in  economy.  Therefore, 
a  record  should  be  made  of  any  solution  reached  in  regard  to 
these  questions.  Almost  every  problem  has  more  than  one 
satisfactory  method  of  solution.  The  multiplication  of  solu- 
tions, however,  particularly  in  manufacturing,  is  a  hindrance 
to  team  work.  For  example,  if  the  foreman  of  one  department 
uses  one  solution  of  a  problem,  while  the  foreman  of  another 
department  who  performs  succeeding  operations  on  the  same  or 

105 


106  INTERCHANGEABLE   MANUFACTURING 

companion  parts  arrives  independently  at  another  solution  and 
uses  it,  the  final  results  may  be  chaos;  whereas,  if  each  solution 
is  recorded,  whenever  or  wherever  made,  an  opportunity  is 
created  to  check  these  solutions  against  each  other,  thus  making 
possible  the  elimination  of  inconsistencies  at  an  early  stage  of 
the  work.  This  practice  will  aid  greatly  in  promoting  team- 
work, and  thereby  eliminate  many  misunderstandings. 

Specifications.  Specifications,  in  their  broadest  sense,  in- 
clude the  solutions  of  all  the  three  problems  mentioned.  This 
information  may  be  compiled  and  recorded  in  one  place  or  it 
may  be  scattered  throughout  the  plant.  In  general,  if  the  entire 
control  of  the  design  and  manufacture  of  a  commodity  is  held 
in  one  plant,  the  compiling  and  assembling  of  much  of  this 
information  may  be  of  doubtful  value.  As  long  as  it  is  on  record 
somewhere  and  available  when  needed,  that'  is  sufficient.  On 
the  other  hand,  if  the  control  of  the  design  rests  with  one  organi- 
zation, the  control  of  the  production  with  another,  while  the 
control  of  the  final  inspection  is  distinct  from  either  of  the  two 
foregoing  establishments,  reasonably  complete  specifications  are 
imperative  if  economical  and  expeditious  production  is  to  be 
obtained. 

Specifications  thus  defined  include  component  drawings. 
For  purposes  of  discussion,  however,  component  drawings  and 
specifications  will  be  considered  as  distinct.  In  this  case  the 
specifications  are  supplementary  to  the  component  drawings 
and  include  all  information  which  is  not  given  on  these  drawings. 

Function  and  Essential  Requirements  of  Product.  The  com- 
ponent drawings  consist  of  pictures  of  the  parts,  statements  of 
the  physical  dimensions  required,  and  usually  specifications  of 
the  material  to  be  employed.  By  themselves  they  only  partially 
solve  the  first  of  the  major  problems  noted  previously.  They 
tell  little  or  nothing  of  the  object  of  the  commodity.  They  state 
requirements,  but  give  no  reasons  therefor.  Thus,  the  first 
function  of  the  specifications  is  to  state  briefly  the  purpose  of 
the  mechanism  and  its  functional  requirements.  The  preceding 
chapter,  " Practice  in  Making  Component  Drawings,"  indicates 
the  lines  which  specifications  should  follow. 


ECONOMICAL  PRODUCTION  107 

A  second  function  of  the  specifications  is  to  indicate  the 
quality  of  workmanship  desired.  The  extent  of  the  tolerances 
given  on  the  drawings  indicates,  to  a  certain  degree,  the  proper 
character  of  the  finished  surfaces.  The  specifications  should 
supplement  this  information  by  stating  not  only  desired  results, 
but  also  reasons  therefor.  The  preceding  chapter  previously 
referred  to  illustrates  this  practice.  To  a  certain  extent,  per- 
haps, many  of  the  conditions  discussed  there  are  so  obvious  as 
to  need  no  mention,  yet  no  harm  is  done  by  being  explicit. 

Another  subject  to  be  included  in  the  specifications  is  the 
matter  of  the  materials  to  be  employed,  and  their  nature,  com- 
position, and  ultimate  use.  When  standardized  material  is 
used,  this  can  be  called  for  directly  on  the  component  drawing, 
together  with  the  proper  heat-treatment.  It  is  of  interest  to 
note  that  the  Society  of  Automotive  Engineers  has  done  much 
valuable  work  in  establishing  standard  specifications  and 
methods  of  heat- treatment  for  nearly  every  kind  of  material 
used  in  automobile  construction.  The  adoption  of  such  stand- 
ards greatly  simplifies  the  provision  of  proper  component  draw- 
ings and  specifications.  In  those  cases  where  standardized 
material  cannot  be  used,  the  specifications  should  give  all  perti- 
nent information  to  enable  the  proper  material  to  be  secured. 
For  preservative  finishes  the  drawings  or  specifications  should 
give  complete  information  as  to  nature,  need,  and  use. 

When  the  component  drawings  have  been  completed  and  the 
specifications  have  reached  this  stage,  the  first  important  ele- 
ments of  the  work  are  established.  Until  this  is  accomplished, 
those  responsible  for  the  manufacturing  design  have  not  done 
all  in  their  power  to  secure  the  economical  production  of  dupli- 
cate mechanisms  in  large  quantities.  Undoubtedly,  many 
minor  revisions  will  be  required  before  the  proper  solutions  of 
the  succeeding  problems  will  be  found.  But  without  the  fore- 
going information,  such  revisions  cannot  be  made  intelligently. 
Furthermore,  this  information,  in  most  cases,  will  point  the 
way  to  a  simple  and  direct  solution  of  the  succeeding  problems. 
"Well  begun  is  half  done"  was  never  nearer  the  truth  than  in 
connection  with  interchangeable  manufacturing. 


108  INTERCHANGEABLE   MANUFACTURING 

Specific  Manufacturing  Data.  We  will  now  consider  the 
solution  of  the  second  major  problem  —  the  development  of 
suitable  manufacturing  methods  and  facilities.  The  first  step 
to  be  taken  is  to  make  up  the  operation  lists  for  every  compo- 
nent, noting  in  detail  the  type  of  machine,  fixture,  and  tool 
required.  It  is  of  great  assistance  in  many  cases  to  develop 
concurrently  the  operation  drawings,  indicating  on  them  the 
work  to  be  performed  at  each  operation.  In  addition,  it  is  a 
good  plan  to  include  in  this  part  of  the  work  an  estimate  of  the 
production  time  on  each  operation.  This  information  is  neces- 
sary to  establish  the  amount  of  equipment  required  and  also  to 
make  a  comparison,  when  desired,  of  the  economy  of  several 
methods. 

This  information  should  be  revised  and  kept  up-to-date  after 
production  is  under  way.  This  furnishes  valuable  data  for 
estimating  on  new  commodities  and  facilitates  comparison  be- 
tween the  costs  of  different  methods  of  manufacturing.  Such 
information  is  invaluable  when  it  becomes  necessary  to  call  on 
outside  plants  to  assist  in  obtaining  greater  production.  It 
need  not  be  bound  together  with  other  parts  of  the  specifica- 
tions, but  it  should  be  in  such  shape  that  it  can  be  quickly 
found  and  readily  applied.  The  foregoing  information  serves 
as  the  basis  for  designing  the  special  manufacturing  equipment 
necessary  as  well  as  for  arranging  the  manufacturing  depart- 
ments so  that  the  component  parts  can  be  produced  rapidly  and 
efficiently. 

This  second  problem  is  seldom  or  never  fully  solved.  Im- 
proved methods  are  being  devised  constantly,  and  these  intro- 
duce new  factors  into  old  problems.  Even  greater  care  must 
be  exercised  in  adopting  a  new  method  on  work  already  in 
process  of  production  than  is  required  in  adopting  the  original 
methods,  because  in  these  cases,  ultimate  economy  requires 
that  such  changes  result  in  a  saving  which  will  pay  for  the  dis- 
carded equipment  as  well  as  for  the  new.  The  effect  of  a  possible 
interruption  in  production  must  also  be  carefully  considered. 

The  production  records  of  the  manufacturing  equipment 
should  be  so  kept  that  it  will  be  always  possible  to  trace  back 


ECONOMICAL  PRODUCTION  1 09 

through  every  change  in  equipment  and  make  direct  compari- 
sons between  the  results  obtained  by  each  method.  At  the 
same  time,  these  records  should  be  so  simple  as  not  to  entail 
unnecessary  clerical  expense.  Whenever  a  change  is  made,  the 
reason  for  making  it  should  be  on  record.  All  these  data  furnish 
information  which  cannot  be  secured  in  any  other  way.  As  a 
matter  of  fact,  many  plants  keep  a  complete  record  of  changes, 
but  do  not  provide  this  class  of  information  when  the  produc- 
tion of  an  entirely  new  mechanism  is  undertaken. 

General  Manufacturing  Data.  In  addition  to  the  specific  in- 
formation required  for  each  individual  part  and  each  assembled 
mechanism,  there  is  a  vast  amount  of  general  data  which  must 
be  had  before  decisions  as  to  the  economy  of  different  methods 
of  manufacture  can  be  made  with  certainty.  Much  of  this 
information  should  be  available  from  the  cost  department 
records,  but,  in  most  cases,  these  records  are  kept  merely  for 
accounting  purposes  and  their  use  as  engineering  data  often 
gives  incorrect  results. 

Factory  Cost  of  Production.  It  is  not  the  purpose  to  outline 
here  a  new  system  of  cost-accounting.  A  discussion  of  some  of 
the  factors  entering  into  the  factory  costs  of  production,  how- 
ever, is  necessary  to  indicate  the  character  of  the  information 
needed  to  promote  economical  production.  For  the  purpose  of 
simpler  accounting,  it  is  often  customary  to  prorate  the  entire 
amount  of  indirect  or  overhead  charges  against  the  total  output 
of  the  plant,  distributing  them  according  to  the  direct  labor 
costs.  From  the  accountant's  viewpoint,  this  method  is  correct. 
If  the  product  of  the  plant  consists  of  one  simple  specialized 
article,  such  a  method  of  accounting  undoubtedly  gives  suffi- 
cient data  for  general  engineering  purposes.  On  the  other 
hand,  if  the  products  are  varied,  or  if  the  productive  operations 
are  subdivided  into  elementary  operations,  performed  in  vari- 
ous departments,  the  data  so  collected  are  incomplete  and  mis- 
leading for  engineering  purposes,  because  the  direct  labor  cost 
alone  will  be  the  determining  factor  in  selecting  the  apparent 
economical  methods  of  production.  As  a  matter  of  fact,  this 
direct  labor  cost  is  but  a  small  percentage  of  the  total  cost  of 


IIO  INTERCHANGEABLE   MANUFACTURING 

production.  It  seldom  amounts  to  25  per  cent.  Furthermore, 
as  the  volume  of  business  increases,  the  percentage  of  direct 
labor  charges  decreases.  Thus,  as  the  quantity  of  production 
increases,  the  data  so  obtained  become  more  and  more  unreliable. 

Another  method  of  distributing  the  indirect  expense  consists 
in  establishing  overhead  rates  for  each  department,  prorating 
these  charges  in  proportion  to  the  direct  labor  cost  as  before. 
If  the  departments  are  arranged  to  contain  only  one  type  of 
equipment,  and  to  perform  similar  operations,  the  data  so  ob- 
tained are  valuable,  but  such  an  arrangement  of  machines  and 
operations  is  seldom  possible  or  desirable.  Different  types  of 
work  creep  into  a  department.  When  this  condition  exists,  the 
information  obtained  from  the  use  of  a  departmental  overhead 
will  again  lead  to  false  conclusions.  Such  a  condition  will 
cause  manufacturing  methods  that  are  not  economical  to  be 
accepted. 

Certain  types  of  equipment  are  always  duplicated  to  some 
extent  in  several  departments.  All  other  things  being  equal, 
the  cost  of  duplicate  operations  on  duplicate  equipment  is 
identical  regardless  of  the  physical  location  in  the  plant.  But 
with  the  use  of  departmental  overhead  charges,  the  book  costs 
will  show  otherwise.  For  example,  in  one  plant  a  sheet-metal 
part  required  a  foot-press  operation  between  two  power-press 
operations.  Foot  presses  were  available  in  two  departments: 
the  power-press  department  with  an  overhead  charge  of  150  per 
cent,  and  a  sub-assembly  department  in  a  distant  part  of  the 
factory  with  an  overhead  charge  of  only  50  per  cent.  The 
original  operation  list  assigned  all  three  operations  to  the  power- 
press  department  to  eliminate  unnecessary  trucking  and  transfer. 
This  was  changed  so  that  the  second  operation  would  be  per- 
formed in  the  other  department  because  of  the  lower  overhead 
there.  Actually,  this  last  method  cost  more  than  the  first 
because  of  the  trucking  and  transfers  back  and  forth,  but  be- 
cause the  book  records  showed  a  higher  cost  for  the  first  method, 
it  was  disapproved  despite  all  arguments.  This  is  not  an  ex- 
treme case.  Similar  conditions  exist  in  the  majority  of  manu- 
facturing plants. 


ECONOMICAL  PRODUCTION  III 

It  is  realized  that  book  costs  and  actual  costs  are  not  identical. 
To  obtain  such  accuracy  would  entail  a  system  so  complex  and 
elaborate  that  its  cost  alone  would  overbalance  all  other  ex- 
penses. Yet  some  simple  way  must  be  found  to  give  more  nearly 
true  costs  of  production  in  order  to  promote  true  economy  of 
manufacture.  The  direct  labor  and  direct  material  costs  are 
readily  obtained.  Most  accounting  methods  apply  these 
charges  directly  against  the  individual  parts,  which  is  the 
proper  distribution.  But  this,  in  most  cases,  disposes  of  less 
than  half  of  the  total  cost  of  production.  Indirect  expenses  are 
not  only  the  most  difficult  to  distribute  equally,  but  also  involve 
the  larger  amount  of  the  costs.  The  total  amount  of  these 
charges  can  be  easily  determined.  This  is  purely  a  matter  of 
bookkeeping.  Their  equitable  distribution,  however,  is  more 
an  engineering  than  an  accounting  problem. 

Distribution  of  Indirect  Factory  Expenses.  There  are  three 
main  factors  to  which  most  of  these  indirect  expenses  can  be 
logically  applied:  First,  the  direct  labor;  second,  the  general 
productive  equipment;  third,  the  component  parts  themselves. 
There  are  also  a  number  of  other  indirect  expenses  which  must 
be  charged  to  the  general  factory  expense.  As  these  are  rela- 
tively few,  they  can  be  arbitrarily  distributed  over  the  entire 
product  without  affecting  the  value  of  the  data  sought.  Even- 
tually, of  course,  the  product  must  carry  them  all.  The  great 
problem  is  to  distribute  them  simply  and  properly. 

If  the  attempt  is  made  to  apply  all  indirect  charges  to  any 
one  of  the  above  factors,  many  economic  errors  will  result. 
Attempts  have  been  made  to  carry  them  all  on  the  direct  labor 
factor  with  far  from  satisfactory  results.  Neither  can  they  all 
be  applied  to  the  equipment  factor  with  any  better  results. 
Each  factor  must  bear  only  its  own  indirect  expenses.  In  order 
to  determine  where  each  indirect  expense  belongs,  a  process  of 
elimination  should  be  adopted.  Without  such  a  factor,  would 
this  expense  exist?  If  the  expense  remains  after  direct  labor, 
general  productive  equipment,  and  individual  components  are 
eliminated,  it  belongs  to  the  general  factory  expenses  or  factory 
management. 


112  INTERCHANGEABLE   MANUFACTURING 

Expenses  Due  to  Direct  Labor.  Let  us  first  consider  the 
indirect  expenses  due  to  direct  labor.  Hereafter,  for  the  sake 
of  brevity,  direct  labor  will  be  referred  to  as  labor. 

Cost  of  Supervision.  One  charge  against  labor  is  the  cost 
of  supervision  as  represented  by  the  salary  of  the  foremen  and 
their  assistants.  The  number  of  these  depends  chiefly  on  the 
number  of  men  to  be  controlled.  This  charge  could  be  prorated 
against  the  number  of  men  employed.  Such  a  method  might, 
in  extreme  cases,  be  erroneous  because  the  higher-priced  men 
should  require  less  supervision.  Although,  in  such  cases,  it 
might  be  more  logical  to  proportion  this  charge  in  an  inverse 
ratio  to  the  wages  of  the  men,  the  clerical  work  necessary  to 
accomplish  this  would  cost  more  than  the  information  would 
be  worth. 

Making  up  Payroll.  The  cost  of  time-keeping  and  making 
up  the  payroll  logically  belongs  to  the  labor  factor.  Eliminate 
labor  and  there  is  no  payroll  to  make  up.  This  should  also  be 
distributed  on  the  basis  of  the  number  of  men  employed,  as  it 
costs  as  much  to  make  up  the  pay  account  of  a  man  getting 
fifteen  dollars  a  week  as  it  does  to  make  up  the  pay  account  of 
a  man  getting  thirty  dollars.  Here  again,  the  indirect  expense 
of  high-priced  labor  is  proportionately  less  than  that  of  lower- 
priced  labor. 

Employment  Department.  The  cost  of  the  employment  de- 
partment also  belongs  to  the  labor  factor.  Eliminate  labor  and 
there  is  no  further  need  of  an  employment  bureau.  These 
charges  should  also  be  distributed  on  the  basis  of  the  number  of 
men  employed,  for  it  costs  as  much  to  hire  one  man  as  another. 
In  many  cases  the  higher-priced  men  are  the  most  reliable  — 
not  always,  of  course,  as  so  many  other  conditions  enter  into 
this  —  and  thus  it  is  possible  that  a  closer  result  would  be 
obtained  by  distributing  the  cost  in  an  inverse  proportion  to 
the  wages  of  the  men. 

Educational  Department.  Wherever  personnel  or  educational 
departments  are  established  or  other  similar  departments  the 
objects  of  which  are  to  promote  cooperation  between  the  em- 
ployer and  employe  to  their  mutual  advantage,  all  expense 


ECONOMICAL  PRODUCTION  113 

incurred  should  be  charged  against  labor.  It  is  extremely  diffi- 
cult to  analyze  such  charges  accurately  —  so  much  depends  upon 
the  nature  of  the  activities  of  such  departments  and  upon  the 
character  of  the  persons  with  whom  they  deal.  Therefore  it 
might  be  best  to  distribute  these  charges  also  on  the  basis  of 
the  number  of  men  employed  so  as  to  simplify  the  accounting 
by  prorating  all  labor  charges  in  a  uniform  manner. 

Maintaining  Health  and  Safety  of  Employes.  All  charges  for 
installing  safety  devices,  fire  escapes,  improved  sanitary  equip- 
ment, for  heating  and  lighting,  and  other  similar  expenses  neces- 
sary for  maintaining  the  health  and  safety  of  the  employes  as 
required  by  law  or  promoted  by  the  dictates  of  humanity,  belong 
to  the  labor  factor.  However,  some  of  these  expenses  might  be 
included  in  other  general  items  —  fire  escapes  may  be  included 
in  the  cost  of  the  buildings,  for  example  —  and  it  may  not  be 
possible  or  feasible  to  isolate  them.  Those  few  cases  where  it 
is  not  practicable  to  apply  expenses  directly  where  they  logi- 
cally belong  will  not  affect  the  values  of  the  final  data  much. 

It  should  offer  no  great  accounting  difficulties  to  isolate  the 
majority  of  the  expenses  enumerated  and  all  other  kindred 
items.  This  is  all  that  the  accountant  would  necessarily  do. 
The  total  amount  so  determined  could  be  prorated  on  the  basis 
of  the  average  number  of  men  engaged  in  actual  production, 
and  thus  give  the  labor  overhead.  This  would  be  used  as  a 
constant  for  a  predetermined  period  in  a  similar  manner  to  the 
usual  overhead  charges,  and  would  be  close  enough  for  all  prac- 
tical purposes.  Of  course,  it  is  evident  that  this  labor  overhead 
fluctuates  constantly,  but  as  these  data  are  for  the  use  of  engi- 
neers and  not  accountants,  an  exact  balance  is  not  essential. 
As  a  matter  of  fact,  the  use  of  a  general  overhead  burden  does 
not  give  an  exact  balance.  A  comparison  between  estimated 
results  and  the  accountants'  records  will  show  how  wise  a  use 
has  been  made  of  this  information.  This  direct  labor  overhead 
is  a  valuable  factor  in  determining  economical  methods  of  pro- 
duction. If  the  general  type  of  labor  employed  differed  to  any 
great  extent  in  the  various  departments,  a  separate  labor  over- 
head could  be  established  for  each  department. 


114  INTERCHANGEABLE   MANUFACTURING 

Machine-hour  Rate.  Next  will  be  considered  the  indirect 
charges  that  belong  to  the  general  productive  equipment  which, 
prorated  on  an  hourly  basis,  will  be  called  the  machine-hour  rate. 

Interest  on  Investment,  Depreciation  and  Insurance.  The  first 
items  are  the  interest  on  the  investment,  depreciation,  and 
insurance.  These  are  relatively  simple  to  determine.  Their 
maintenance  charges  belong  here  also.  When  possible,  these 
should  be  applied  to  the  particular  types  of  machines.  The 
many  petty  items  would  be  distributed  over  the  entire  equip- 
ment. Having  the  machinery,  there  must  be  a  place  to  put  it. 
The  majority  of  the  factory  space  is  utilized  by  productive 
equipment.  It  would  seem,  therefore,  that  all  the  fixed  plant 
charges  would  be  best  distributed  here.  This  could  be  done 
proportionately  to  the  average  floor  space  required  for  operat- 
ing each  type  of  machine. 

Power  Charges.  The  power  charges  would  also  be  included 
in  the  machine-hour  rate.  One  cannot  go  into  the  exact  dis- 
tribution of  this  factor  without  incurring  great  expense  in 
power  tests  —  tests  that  become  valueless  as  the  machining 
cuts  vary  from  light  to  heavy,  short  to  long.  An  approximation 
could  be  made  which  would  determine  the  value  of  this  factor 
close  enough  for  practical  purposes.  A  few  power  tests  could 
be  made  to  advantage  for  the  purpose  of  securing  data  to  assist 
in  the  proper  distribution  of  all  power  costs.  The  cost  of  belting 
and  lubricating  oils,  etc.,  could  also  be  included  in  the  total 
power  charges.  Such  a  plan  would  insure  an  equitable  and 
simple  method  of  distributing  these. 

Non-productive  Time  of  Machines.  There  is  always  a  certain 
amount  of  idle  time  for  any  machine,  no  matter  how  well  the 
work  is  planned.  The  amount  of  this  normal  non-productive 
time  varies  for  different  types  of  machines.  For  example,  an 
automatic  screw  machine  —  as  efficient  a  productive  machine 
as  there  is  —  is  idle  on  an  average  of  one  hour  in  five.  This  idle 
time  is  caused  by  the  necessity  of  adjusting  tools,  oiling,  chang- 
ing bars,  etc.  Therefore,  the  actual  productive  time  of  this 
machine  is  80  per  cent  of  its  total  operating  time.  In  the  print- 
ing trades,  where  a  complete  system  of  machine-hour  costs  has 


ECONOMICAL  PRODUCTION  115 

been  developed,  65  per  cent  is  considered  as  a  normal  average  of 
productive  time.  The  normal  percentage  of  non-productive 
time  should  be  estimated  as  closely  as  possible  and  included 
in  the  machine-hour  rate. 

Lack  of  Work  or  of  Labor.  There  is,  however,  another  source 
of  idle  time  to  be  considered.  This  may  be  lack  of  work  or  lack 
of  labor.  If  it  is  lack  of  labor,  it  should  logically  be  charged 
against  the  employment  department.  Lack  of  work  may,  of 
course,  be  due  to  one  of  several  causes.  If  due  to  lack  of  busi- 
ness, it  could  be  charged  against  the  sales  department,  while  if 
due  to  poor  planning,  it  belongs  to  the  general  factory  expense. 
For  simpler  accounting,  abnormal  idle  time  might  best  be 
included  in  the  general  factory  expense. 

Keeping  of  Records.  The  shop  office  carries  records  of  the 
machinery,  such  as  inventory  lists  giving  the  original  value, 
depreciation,  locations,  etc.  The  results  of  using  machine-hour 
rates  would  be,  to  all  intents  and  purposes,  the  creation  of  a 
payroll  for  the  various  machines.  All  this  would  require  a 
certain  amount  of  clerical  work,  the  cost  of  which  should  be 
prorated  against  the  machines. 

The  accountant  would  carry  accounts  showing  only  the 
total  amounts  spent  for  power,  belts,  lubricating  oil,  fixed  plant 
charges,  fixed  charges  on  general  manufacturing  equipment, 
labor  costs  for  machine  records,  etc.  These  totals  would  be 
prorated  as  just  discussed  to  establish  a  constant  machine-hour 
rate  for  each  type  of  equipment,  which  would  be  simple  to  apply. 
Such  values  would  be  adjusted  periodically  as  required.  Here, 
again,  the  successive  reports  of  the  accountant  would  make 
apparent  how  wise  a  use  had  been  made  of  the  information  so 
gathered. 

The  mechanical  manufacturing  industries  would  find  it  well 
worth  their  while  to  follow,  in  one  respect  at  least,  the  example 
of  the  printing  trades.  The  problem  of  the  actual  cost  of  produc- 
tion has  been  studied  by  them  on  a  cooperative  basis.  For 
several  years,  one  of  their  trade  journals  has  been  collecting  cost 
data  from  many  plants  throughout  the  country.  The  informa- 
tion so  willingly  given  has  been  compiled  and  analyzed  for  the 


Il6  INTERCHANGEABLE   MANUFACTURING 

benefit  of  all.  As  a  result,  a  complete  series  of  average  machine- 
hour  costs  has  been  developed,  the  use  of  which  gives  results 
very  close  to  the  actual  balances  of  the  accountants.  This 
data  is  kept  up  to  date  and  corrected  values  are  distributed 
periodically.  This  information  is  invaluable  for  estimating  and 
other  planning  purposes. 

Product  Overhead.  The  indirect  expenses  due  to  the  prod- 
uct itself  will  now  be  considered.  The  following  method  of 
distributing  indirect  costs  is  a  radical  departure  from  any  estab- 
lished practice,  yet  their  logical  distribution  leaves  no  other 
path  open.  There  are  two  classes  of  these  charges;  those  which 
can  be  applied  to  specific  component  parts  and  those  which 
apply  to  the  whole  product  in  general.  For  the  sake  of  brevity, 
those  of  the  first  type  may  be  called  specific  product  charges 
and  those  of  the  second,  general  product  charges. 

Specific  Product  Charges.  The  most  important  specific  prod- 
uct charge  and  the  one  most  readily  isolated  is  that  for  jigs, 
fixtures,  special  tools,  and  gages.  These  certainly  belong  to 
specific  parts.  The  adoption  of  this  practice  will  soon  develop 
a  valuable  record.  Tt  will  then  be  possible  to  obtain  —  and 
apply  directly  where  it  belongs  —  the  cost  of  making  changes  in 
the  design  and  methods  of  manufacturing  of  the  various  parts. 
Then  only  will  it  be  possible  to  determine  whether  or  not  such 
changes  result  in  an  economic  gain.  The  writer  believes  that 
frequent  changes  of  this  sort  are  altogether  too  common  in 
American  manufacturing  practice.  Judging  from  personal  ex- 
perience during  the  past  ten  years,  the  majority  of  such  changes 
are  the  results  of  mental  laziness.  When  the  first  important 
difficulty  appears,  the  issue  is  avoided  instead  of  being  carried 
through  to  its  logical  conclusion.  It  is  the  easiest  thing  in  the 
world  to  try  to  make  a  part  in  a  different  way  from  the  one 
originally  planned;  but  by  so  doing  one  set  of  difficulties  with 
which  we  are  somewhat  familiar  is  merely  substituted  by  others 
with  which  we  have  had  no  previous  experience,  and,  in  the 
end,  no  progress  has  been  made.  The  writer  feels  safe  in  stating 
that  not  less  than  seventy-five  per  cent  of  the  changes  made 
are  attempts  to  avoid  trouble  which  is  not  eventually  escaped, 


ECONOMICAL  PRODUCTION  1 17 

and  that  this  percentage  of  the  cost  of  changes  represents  a 
total  economic  loss.  A  method  of  distributing  indirect  expenses 
along  the  lines  indicated  will  expose  such  conditions  as  nothing 
else  will. 

Another  specific  product  charge  is  the  cost  of  constructing 
special  machines  for  specific  parts.  This  is  logically  included  in 
the  jig  and  fixture  costs.  When  the  special  machine  is  used  on 
several  parts,  these  expenses,  of  course,  will  be  classified  with 
other  general  machinery  items.  Its  normal  non-productive 
time,  however,  is  usually  large. 

The  loss  resulting  from  scrapped  work  is  another  specific 
product  charge.  Some  parts  are  delicate  and  difficult  to  ma- 
chine and  this  results  in  a  high  percentage  of  scrap.  Others  are 
simple  and  more  rapidly  produced  with  little  or  no  scrap.  It 
is  manifestly  unfair  to  distribute  this  item  of  expense  over  the 
product  as  a  whole  because  this  will  give  an  entirely  wrong  idea 
in  regard  to  the  economy  of  the  simpler  and  sturdier  designs. 
Often,  when  the  design  is  changed,  a  large  number  of  finished 
parts  are  scrapped  or  reworked.  All  such  expenses  should  be 
charged  to  specific  pieces  when  possible.  There  will,  of  course, 
be  some  credit  items,  due  to  salvage.  If  it  should  be  difficult 
and  expensive  to  distribute  these  salvage  credits  specifically, 
they  could  be  credited  to  a  general  scrap  account  and  applied 
proportionately  to  the  cost  of  the  scrap  charged  against  each 
part.  Any  inaccuracy  resulting  from  this  procedure  would  have 
little  effect  on  the  value  of  the  final  result. 

General  Product  Charges.  Most  of  the  other  expenses  in- 
curred by  the  product  are  general  product  charges.  Among 
them  would  be  the  cost  of  trucking,  including  the  interest  on 
the  investment  of  the  trucks,  elevators,  etc.,  depreciation,  and 
their  maintenance  costs.  The  expenses  of  shop  rearrangements 
also  belong  with  the  general  product  charges,  as  these  altera- 
tions are  made  to  facilitate  production.  If,  however,  they  can 
be  charged  directly  to  specific  component  parts,  they  should  be 
so  distributed.  The  majority  of  the  factory  office  expenses 
would  be  included  in  the  general  product  charges.  This  would 
include  the  cost-keeping,  production  records,  engineering  and 


Il8  INTERCHANGEABLE   MANUFACTURING 

experimental  work,  etc.  All  these  general  product  charges  could 
be  distributed  in  conjunction  with  the  general  factory  expenses. 
This  method  would  be  as  equitable  as  any. 

Clerical  and  Accounting  Work.  The  clerical  and  accounting 
work  which  such  a  procedure  would  entail  will  now  be  con- 
sidered. First,  the  accountant  must  arrange  his  books  so  as  to 
separate  all  expenses  due  to  direct  labor.  This  would  be  only 
a  single  account.  Next  he  would  open  another  account  to 
carry  all  the  expenses  making  up  the  machine-hour  rate  of  the 
general  productive  equipment.  Another  account  would  be 
opened  for  the  indirect  charges  due  to  the  product.  The  in- 
direct charges  caused  by  the  purchase  and  handling  of  the 
material  have  not  been  previously  mentioned.  These  would  be 
handled  in  the  same  manner  as  the  other  indirect  expenses,  and 
distributed  over  the  direct  material.  The  accountant  would 
carry  tjiis  account,  and  one  to  cover  the  general  factory  expenses 
which  are  not  distributed  elsewhere.  These  would  be  all  of  the 
indirect  factory  expense  accounts  which  he  would  require.  He 
would  also  carry  direct  labor  and  material  accounts.  The  sum 
of  these  accounts  would  represent  the  factory  cost  of  production 
of  the  work  produced  during  a  given  period.  This,  balanced 
against  the  value  of  the  output  would  represent  the  gain  or  loss 
during  the  specified  period.  This  is  the  essential  information  in 
which  the  stockholders  and  directors  of  a  plant  are  interested. 

The  factory  cost  department  would  carry  independent  ac- 
counts along  entirely  different  lines.  These  should  give  detailed 
information  in  regard  to  the  cost  of  each  component.  They 
need  not  necessarily  check  exactly  with  the  accountant's  records. 
The  constants  which  are  used  by  the  factory  should  be  so  estab- 
lished as  to  make  the  total  of  these  records  a  small  percentage 
higher  than  the  accountant's.  A  factor  of  safety  of,  say,  5  per 
cent,  should  be  included  in  the  various  constants.  The  direct 
labor  and  material  charges  are  the  only  ones  which  should  check 
absolutely  in  both  sets  of  records,  while  all  the  others  need 
check  only  within  the  limits  of  the  factor  of  safety  established. 
The  various  constants  would,  necessarily,  be  adjusted  from 
time  to  time  to  bring  this  result. 


ECONOMICAL  PRODUCTION  119 

The  factory  cost  department  would  carry  an  account  for 
each  separate  component,  including  sub-assemblies  and  com- 
plete mechanisms.  Records  must  be  kept  on  each  of  these 
accounts,  showing  the  direct  labor,  direct  material,  machine- 
hours,  special  equipment,  and  scrap  charges.  To  this  would 
be  added  the  proper  direct  labor  overhead,  direct  material 
overhead,  machine-hour  rates,  general  product  overheads  and 
general  factory  expense.  These  last  would  be  specified  con- 
stants. The  comparison  of  these  shop  records  with  the  account- 
ants' records  would  prove,  first,  the  relative  accuracy  of  the 
established  constants,  and,  second,  how  effective  a  use  had 
been  made  of  the  data  that  was  so  collected. 

If  it  were  possible  for  several  manufacturing  concerns  to  adopt 
a  method  of  factory  cost-keeping  along  these  lines  and  to  com- 
pare from  time  to  time,  not  the  details  but  certain  of  the  result- 
ant factors,  much  valuable  information  would  be  secured  — 
information  which  would  not  in  the  least  reveal  any  of  the  con- 
fidential facts  of  the  business  but  would  uncover  many  economic 
truths  as  yet  undiscovered.  For  example,  it  is  of  the  utmost 
value  to  determine  what  the  normal  direct  labor  burden  should 
be.  The  different  plants  involved  in  such  an  undertaking  need 
not  be  engaged  in  the  manufacture  of  the  same  products,  and 
their  methods  of  manufacturing  might  differ  in  many  particu- 
lars, yet  from  data  so  obtained  the  normal  cost  of  this  factor 
could  be  determined  closely.  These  plants  would  thus  estab- 
lish a  standard  with  which  to  compare  their  costs,  giving  them 
either  the  gratifying  knowledge  that  their  labor  burden  was 
normal  or  else  a  warning  of  conditions  that  should  be  corrected. 
The  same  is  true  in  regard  to  the  machine-hour  rates.  With 
cooperation  between  a  large  number  of  plants,  an  accurate  series 
of  values  could  be  established  for  these  rates.  Accuracy  or 
precision  in  manufacturing  requires  established  standards.  Why 
should  not  the  same  principle  be  applied  to  other  problems? 
If  relative  standards  for  machine-hour  rates  and  direct  labor 
overhead  are  established,  for  example,  the  same  progress  can 
be  expected  in  these  respects  as  has  been  made  in  mechanical 
work  since  the  establishment  of  physical  standards.  It  may 


120  INTERCHANGEABLE   MANUFACTURING 

be  a  long  and  slow  process,  but  there  is  sure  to  be  improvement 
during  its  development.  The  product  factors  would  be  of  little 
value  for  general  comparisons.  Their  nature  prevents  their 
economic  use  for  this  purpose. 

Inspection  and  Testing.  The  third  major  problem  —  the 
development  of  inspection  methods  and  facilities  —  should  be 
solved  to  a  great  extent  in  conjunction  with  the  development 
of  the  other  manufacturing  equipment.  The  inspection  opera- 
tions and  necessary  gages  should  appear  in  their  proper  place 
on  the  operation  lists.  In  some  cases,  these  lists  should  be 
supplemented  by  a  more  detailed  description  of  the  methods  of 
inspection.  This  is  unnecessary  for  limit  gages  measuring  ele- 
mentary surfaces.  It  is  required,  however,  in  connection  with 
the  use  of  functional  and  other  gages  for  many  composite  sur- 
faces. Such  information  should  be  included  in  the  specifications 
as  an  integral  part  of  the  description  and  requirements  of  each 
component  part. 

Specific  and  General  Information.  It  is  obvious,  then,  that 
specifications  may  be  divided  into  two  main  divisions.  The 
first  division  contains  the  specific  information  required  in  the 
production  of  particular  commodities.  The  preceding  chapter 
gives  a  good  illustration,  as  far  as  the  requirements  of  dimen- 
sions and  surfaces  are  concerned,  of  the  nature  of  specifications 
of  this  class.  Operation  lists,  material  specifications,  inspection 
requirements,  etc.,  are  needed  to  make  them  complete.  In 
this  way,  the  efforts  of  all  will  be  directed  along  similar  lines, 
the  exacting  requirements  will  receive  the  greatest  attention 
—  and  this  they  should  always  receive  —  while  those  of  lesser 
importance  will  be  treated  accordingly. 

The  second  division  consists  of  that  vast  amount  of  general 
information  that  is  derived  from  cost  and  production  records, 
"  traditions  of  the  shop,"  and  all  other  general  data  gleaned 
from  every  possible  source  which  applies  equally  to  every  com- 
modity. This  should  not  be  duplicated  in  the  written  specifica- 
tions of  every  individual  commodity,  but  should  be  gathered 
independently  in  usable  form. 


CHAPTER  VIII 
EQUIPMENT   FOR    INTERCHANGEABLE    MANUFACTURING 

IN  the  preceding  chapters  an  idea  was  given  of  the  informa- 
tion required  for  interchangeable  manufacturing,  desirable  lines 
to  be  followed  in  obtaining  these  data  were  indicated  and  proper 
methods  of  recording  this  information  were  described  and  illus- 
trated. Even  with  the  exercise  of  the  greatest  care  in  this 
preliminary  work,  many  petty  details  will  be  overlooked;  but 
the  design  and  construction  of  the  manufacturing  equipment 
can  be  carried  through  with  expedition  and  confidence  as  soon 
as  this  preliminary  information  is  obtained.  If  time  is  impor- 
tant, the  work  of  designing  and  constructing  this  equipment 
can  be  turned  over  to  a  large  number  of  persons  or  to  several 
different  plants  to  develop  independently.  It  is,  of  course,  essen- 
tial that  a  uniform  method  of  interpreting  drawings  and  toler- 
ances be  used,  and  that  complete  operation  lists  be  furnished. 

These  operation  lists  should  map  out  the  plan  of  action  for 
the  selection  or  design  of  the  equipment.  Fig.  i  shows  a  form 
which  has  proved  satisfactory  in  service.  ,The  preliminary  lists 
should  give  such  descriptions  of  the  special  tools  and  gages  to 
be  made  that  their  design  can  be  readily  developed.  The  final 
lists  may  refer  to  these  tools  and  gages  by  number  only.  If 
these  lists  are  kept  up  to  date,  they  become  a  valuable  key  to 
all  production. 

Selection  of  Machine  Tools.  In  order  to  insure  ultimate 
economy,  the  proper  choice  must  be  made  between  standard  or 
special  machine  tools.  The  amount  and  type  of  available  equip- 
ment affects  this  decision.  In  general,  standard  equipment  is 
advisable,  although  occasionally  the  reverse  is  true.  For  ex- 
ample, in  the  case  of  an  extremely  high  rate  of  production, 
special  automatic  machines  built  to  serve  one  specific  purpose 
prove  more  economical.  In  other  cases,  a  small  special,  single- 


122 


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EQUIPMENT  123 

typewriters;  hence,  to  meet  this  demand,  new  features  have  been 
introduced  on  the  machines,  such  as  tabular  stops,  etc.  The 
possibility  of  such  modifications  must  never  be  overlooked. 
Another  characteristic  of  the  machine  tool  equipment,  there- 
fore, is  its  adaptability.  All  other  factors  being  equal,  that 
machine  which  is  the  most  adaptable  to  other  operations  should 
be  chosen.  In  this  connection,  it  is  of  interest  to  note  that 
single-purpose  machines  are  usually  the  more  adaptable,  and 
furthermore,  that  they  often  prove  more  economical  in  other 


Fig.  2.     Early  Type  of  Manufacturing  Milling  Machine 

ways.  For  instance,  the  writer  knows  of  a  watch  factory  which 
was  engaged  in  making  fuse  bodies  during  the  war.  Some  were 
made  on  semi-automatic  turret  lathes  and  others  were  machined 
on  small  bench  lathes.  The  turret  lathes  practically  completed 
the  parts  in  two  operations,  while  more  than  a  dozen  operations 
were  required  on  the  bench  lathes.  The  parts  produced  on  the 
bench  lathes  were  not  only  more  nearly  identical  than  those 
produced  on  the  turret  lathes,  but  also  cost  less  to  manufacture. 


124  INTERCHANGEABLE   MANUFACTURING 

The  turret  lathes  were  purchased  particularly  for  this  job  while 
the  bench  lathes  had  been  formerly  used  in  turning  watch  cases. 
The  machines  selected  must  be  sufficiently  rigid  to  perform 
their  task.  The  introduction  of  high-speed  steels  for  cutters 
has  created  more  severe  conditions  than  formerly  existed,  and 
the  improvement  of  the  cutters  in  this  respect  has  caused  a 
great  increase  in  the  rigidity  of  many  machine  tools.  It  is  inter- 


Fig.  3.     Modern  Type  of  Manufacturing  Milling  Machine 

esting  to  compare  the  general  construction  of  an  earlier  type 
of  manufacturing  milling  machine  which  is  shown  in  Fig.  2, 
with  that  of  a  more  recent  type  shown  in  Fig.  3. 

From  the  production  standpoint,  the  most  important  factor 
of  the  machine  tool  equipment  is  the  ease  and  facility  with 
which  it  can  be  set  up,  adjusted,  and  operated.  Several  suc- 
cessive operations  on  simple  single-purpose  machines  are  often 


EQUIPMENT 


more  economical  than  a  single  operation  on  a  more  complicated 
machine.  The  actual  production  time  of  the  latter  may  be 
much  less  than  that  of  the  former,  but  when  it  is  stopped  for 
adjustment  and  setting-up,  the  loss  of  production  is  correspond- 
ingly greater.  Furthermore,  the  multiplicity  of  adjustable  parts 
makes  it  necessary  to  stop  for  readjustments  more  frequently. 
It  should  not  be  assumed  from  this  that  single-operation  ma- 
chines are  always  the  most  economical,  because  on  parts  hav- 
ing liberal  tolerances  the  reverse  is  true. 

Design  of  Jigs  and  Fixtures.     Jigs  and  fixtures  are  provided 
to  assist  in  machining  specific  surfaces  on  specific  parts,  and 


171 


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Machinery 


Fig.  4.     Drawing  of  Stud  machined  in  One  Operation  on  a  Screw 
Machine 

their  design  depends  to  a  large  extent  upon  the  design  of  the 
parts  to  be  machined.  There  are,  however,  a  number  of  general 
principles  which  apply  equally-  to  all  types  of  fixtures.  The 
holding  or  register  points  of  tools  and  fixtures  for  all  finishing 
cuts  should  be  identical  with  /those  surfaces  from  which  the 
dimensions  are  given  on  the  component  drawings.  On  roughing 
cuts,  these  holding  points  are  of  lesser  importance,  yet  it  is 
good  practice  to  maintain,  as  far  as  possible,  the  same  register 
points  on  both  roughing  and  finishing  cuts.  The  stud  shown  in 
Fig.  4  is  presented  as  a  simple  example  to  illustrate  this  prin- 
ciple. It  will  be  noted  that  the  dimensions  of  length,  except  the 


126 


INTERCHANGEABLE   MANUFACTURING 


depth  of  the  counterbore,  are  all  given  from  one  end,  that  the 
depth  of  the  counterbore  is  given  from  the  opposite  end,  and 
that  the  part  is  to  be  machined  all  over.  This  part  can  be 
machined  in  a  single  operation  on  a  screw  machine  by  means  of 
the  tools  illustrated  in  Fig.  5. 

Surface  Y  of  the  cutting-off  tool  A  establishes  the  register 
point  for  forming  tool  B  and  facing  tool  C.  Surface  X  of  the 
forming  tool  is  adjusted  in  relation  to  surface  Y  of  the  cutting-off 
tool  so  that  the  length  of  the  stem  on  the  work  will  be  as  given 
on  the  component  drawing.  Surface  Z  of  the  facing  tool  is  also 
adjusted  in  relation  to  surface  Y  of  the  cutting-off  tool  in  order 


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Machinery 


Fig.  5. 


Diagrammatic  Illustration  of  Tools  used  in  machining 
Stud 


to  maintain  the  proper  over-  all  length  of  the  piece.  Counter- 
bore  D  is  provided  with  an  adjustable  stop  E  which  registers 
against  the  forward  end  of  the  stud.  The  contact  surface  of 
this  stop  is  adjusted  in  relation  to  surface  W  of  the  counterbore 
so  as  to  maintain  the  prescribed  depth. 

If  the  bottom  of  this  counterbore  were  located  on  the  com- 
ponent drawing  from  the  end  of  the  stem,  the  stop  on  the  turret 
would  be  adjusted  so  that  surface  W  of  the  counterbore  would 
keep  its  proper  position  in  relation  to  surface  Y  of  the  cutting-off 
tool.  Stops  on  the  cross-slide  carrying  the  forming  tool  control 
the  outside  diameters. 


EQUIPMENT  127 

The  next  question  that  arises  is  the  proper  limit  at  which  to 
set  the  tools.  There  are  four  main  sources  of  variation  to  con- 
sider: First,  the  errors  resulting  from  imperfections  in  the 
machine;  second,  those  caused  by  wear  on  the  cutting  edges  of 
the  tool;  third,  those  due  to  errors  in  the  tools;  and  fourth, 
those  caused  by  improper  setting  of  the  raw  material  in  their 
holding  devices.  In  the  following,  only  those  variations  which 
affect  the  lengths  of  the  stud  will  be  dealt  with. 

Results  of  End  Play  in  the  Machine  Spindle.  Any  end  play 
in  the  spindle  of  the  machine  will  reduce  the  distance  between 
the  surfaces  machined  by  tools  A  and  C,  Fig.  5.  If  the  forming 
and  cutting-off  tools  are  held  rigidly  together,  this  end  play 
will  not  affect  the  distance  between  the  surfaces  machined  by 
tool  A  and  surface  X  on  tool  B.  If,  however,  these  two  tools 
are  independent  of  each  other,  as  shown  in  the  illustration,  this 
end  play  may  cause  a  variation  in  either  direction.  In  the  first 
case  mentioned,  a  male  dimension  is  dealt  with  and  inaccuracies 
in  the  action  of  machine  tools  cause  a  minus  variation  in  such 
a  length  whether  the  cutting  tools  are  independent  of  each  other, 
or  combined  in  a  single  tool.  Similarly  in  the  case  of  a  female 
dimension,  it  is  evident  that  the  variation  will  be  plus.  End 
play  in  the  machine  will  not  affect  the  depth  of  counterbore 
when  the  tool  is  self-registering  as  shown;  thus  a  self -registering 
tool  often  produces  more  accurate  results  than  one  controlled 
by  stops  on  the  machine  itself. 

Wear  on  the  cutting  edges  of  tools  A  and  C  will  increase  the 
distance  between  surfaces  Y  and  Z,  but  if  the  wear  on  surfaces 
Y  and  X  is  uniform,  no  variation  will  develop  from  this  source. 
If  it  is  not  uniform,  the  variation  may  be  in  either  direction. 
As  surface  W  becomes  worn,  the  distance  between  this  surface 
and  the  contact  surface  of  stop  E  decreases,  that  is,  of  course, 
assuming  that  there  is  no  appreciable  wear  on  the  contact  sur- 
face of  the  stop. 

Considering  only  the  first  two  factors,  that  is,  imperfections 
in  the  machines  and  wear  on  the  cutting  edges  of  the  tools,  the 
original  setting  of  facing  tool  C  should  be  as  near  to  the  mini- 
mum limit  as  the  accuracy  of  the  machine  permits;  the  setting 


128  INTERCHANGEABLE   MANUFACTURING 

of  forming  tool  B  should  be  at  the  mean  dimension  until  actual 
practice  demonstrates  a  tendency  to  vary  more  in  one  direc- 
tion than  the  other;  while  the  setting  of  stop  E  on  counterbore 
D  should  be  as  near  to  the  maximum  limit  as  possible.  This 
will  give  the  maximum  time  between  readjustments.  The  con- 
ditions created  by  'errors  in  the  tools  themselves  will  be  dealt 
with  later  in  this  chapter. 

Results  of  Wear  on  Cutting  Edges  of  Tools.  It  is  thus  evi- 
dent that  the  effect  of  imperfections  in  the  operation  of  machine 
tools,  when  the  cutting  tools  are  located  by  stops  on  the  machine, 
is  to  cause  a  plus  variation  on  female  dimensions,  a  minus  varia- 
tion on  male  dimensions,  and  a  plus  and  minus  variation  on 
neuter  dimensions,  such  as  the  horizontal  distance  between 
surfaces  Y  and  X,  which  cannot  be  strictly  classified  as  either 
male  or  female.  In  other  words,  imperfections  in  machine  tools, 
such  as  end  play,  etc.,  cause  additional  metal  to  be  removed. 
In  a  similar  manner,  the  wear  on  the  cutting  edges  of  the  tools 
causes  a  minus  variation  on  female  dimensions,  a  plus  varia- 
tion on  male  dimensions,  and  a  plus  or  minus  variation  on  neuter 
dimensions.  In  the  last  case,  if  the  cutting  edge  of  one  tool 
consistently  wears  faster  than  that  of  the  other,  the  variation 
will  run  in  only  one  direction.  Assuming  that  surface  Y  wears 
faster  than  surface  X,  the  variation  will  be  plus.  If  the  wear 
is  equal,  no  variations  develop  from  this  cause.  In  other  words, 
the  wear  on  the  cutting  edges  of  the  tools  usually  causes  a  varia- 
tion in  the  reverse  direction  from  that  caused  through  the 
presence  of  imperfections  in  the  machine  tools. 

The  same  principles  apply  equally  to  all  types  of  operations 
such  as  turning,  milling,  planing,  grinding,  profiling,  shaping, 
boring,  etc.  In  order  to  establish  the  proper  dimensions  for 
the  holding  and  registering  points  on  tools  and  fixtures,  it  is 
necessary  to  analyze  each  particular  surface  carefully  and  pro- 
ceed accordingly. 

In  the  above  example,  errors  due  to  the  improper  setting  of 
the  stock  in  the  holding  device  are  not  present.  In  the  case 
of  milling,  drilling  and  other  similar  operations,  where  the  parts 
are  handled  singly  instead  of  being  machined  from  bar  stock, 


EQUIPMENT  I2Q 

this  is  one  of  the  most  serious  problems.  Chips  are  apt  to  re- 
main on  the  locating  points  or  the  operator  may  fail  at  times 
to  seat  the  piece  properly  before  clamping.  In  the  case  of  drill- 
ing, this  results  in  the  improper  location  of  the  holes.  In  the 
case  of  other  operations,  it  will  result  in  removing  additional 
stock.  Proper  training  of  the  operator  is  the  only  cure  for  this 
fault.  Correctly  designed  fixtures  greatly  reduce  the  chances 
of  these  errors,  yet,  as  in  other  matters,  it  is  not  possible  to 
make  everything  so  that  it  will  be  entirely  fool-proof. 

Important  Factors  in  Designing  Fixtures.  The  second  im- 
portant factor  of  the  design  of  fixtures  is  their  operation  in  serv- 
ice. The  selection  of  the  proper  locating  points  controls  in  a 
large  measure  the  uniformity  of  the  product.  The  facility  with 
which  these  fixtures  may  be  operated  determines  to  a  great 
extent  the  rate  of  production.  The  direct  labor  cost  of  produc- 
tion is  also  greatly  reduced  with  quick-acting  jigs  and  fixtures. 
On  the  other  hand,  such  equipment  is  often  very  expensive,  and 
therefore  the  total  output  involved  determines  the  amount  of 
money  which  may  be  economically  expended  on  the  equipment. 
In  the  case  of  a  very  small  total  output,  little  or  no  special  equip- 
ment need  be  provided. 

When  continuous  production  is  involved,  every  effort  should 
be  made  in  designing  the  equipment  to  have  it  operate  rapidly. 
It  should  operate  easily  and  with  a  single  motion  of  the  opera- 
tor's hand  if  possible.  Second,  the  fixture  should  open  so  that 
the  part  to  be  removed  is  accessible.  Third,  the  position  of 
such  openings  in  relation  to  the  cutting  tools  should  be  such 
that  there  is  no  danger  to  the  operator.  Whenever  the  operator 
is  required  to  place  his  hand  close  to  the  cutting  tool,  he  nor- 
mally moves  slowly  and  cautiously,  thus  reducing  the  rate  of 
production.  Fourth,  all  exposed  sharp  corners  and  edges  on 
the  fixtures  should  be  eliminated  to  prevent  injury  to  the  opera- 
tor. Fifth,  the  locating  points  should  be  accessible,  to  facilitate 
cleaning  and  the  proper  insertion  of  the  work.  Sixth,  liberal 
chip  clearances  should  be  provided  to  facilitate  cleaning  the 
fixture  and  also  to  prevent  marring  the  machined  surfaces  of 
the  parts  under  the  process  of  manufacture. 


130  INTERCHANGEABLE    MANUFACTURING 

Examples  of  Efficient  Fixture  Design.  Except  for  relatively 
large  parts,  careful  study  will  make  apparent  a  simple  and 
effective  means  of  clamping  the  work  with  a  single  motion  of 
the  operating  handle.  Loose  nuts  and  screws  should  be  avoided 
for  clamping  purposes  whenever  possible,  as  these  are  very  slow 
to  operate.  A  milling  fixture  which  requires  only  a  single 
motion  to  clamp  is  shown  in  Fig.  6.  An  assembly  view  of  this 


Fig.  6.     Design  of  Milling  Fixture  in  which  Three  Pieces  of  Work 
are  clamped  by  a  Single  Motion 

fixture  is  shown  in  Fig.  7,  and  the  details  of  its  construction 
will  be  understood  by  reference  to  this  illustration. 

This  fixture  is  used  when  milling  the  bottom,  right-  and  left- 
hand  sides  of  a  small  forging  in  a  single  operation.  The  forging 
is  first  placed  in  the  position  indicated  at  A  by  dot-and-dash 
lines,  where  one  side  is  milled.  This  piece  is  then  advanced  to 
position  B,  where  the  opposite  side  is  milled.  It  is  then  inserted 
at  C  in  which  position  the  bottom  is  milled.  In  actual  opera- 
tion, when  the  original  piece  is  placed  in  position  B,  a  second 


EQUIPMENT 


piece  is  placed  at  A,  and  when  the  original  piece  is  moved  to 
C}  the  second  piece  is  moved  to  B  and  a  third  piece  is  placed 
at  A.  Thus  three  pieces  are  being  machined  at  the  same  time. 
All  three  parts  are  clamped  by  one  motion  of  the  lever,  D  and  E 
being  fixed  jaws  and  F  and  G  clamping  jaws.  The  cam  which 
is  attached  to  the  operating  lever  draws  back  the  jaw  F  and 
pushes  forward  the  jaw  G.  The  jaw  F  is  designed  so  that  it  can 


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Machinery 


Fig.  7. 


Assembly  Drawing  of   the  Milling  Fixture  illustrated  in 
Fig.  6 


rock,  thus  securing  the  parts  at  A  and  B  with  equal  pressure 
regardless  of  the  variations  in  size  between  the  two  pieces.  This 
particular  construction  permits  clamping  in  three  places  simul- 
taneously with  a  single  motion  of  the  operating  lever.  Modifica- 
tions may  be  made  in  this  design  which  will  permit  clamping  in 
three  or  more  different  directions  when  this  is  desirable  or 
necessary. 


132 


INTERCHANGEABLE   MANUFACTURING 


Inaccessibility  is  one  of  the  most  common  faults  found  with 
jigs  and  fixtures.  Sufficient  clearance  should  always  be  allowed 
to  enable  the  operator  to  remove  the  completed  work  readily 
and  also  to  insert  a  new  part  and  hold  it  in  position  while  clamp- 
ing. Careful  attention  to  this  point  will  greatly  promote  rapid 
production.  Fig.  8  .shows  a  spline  milling  fixture  which  affords 
a  simple  example  of  this  point.  The  construction  of  this  fixture 
is  shown  in  Fig.  9.  Two  pieces  are  held  at  one  time,  the  pieces 


Fig.  8. 


Machine  provided  with  Fixture  used  in  Spline  Milling  in 
which  the  Work  is  Readily  Accessible 


being  clamped  in  position  by  a  leaf.  It  will  be  noted  that  each 
piece  is  clamped  in  two  places  by  a  single  operating  lever.  The 
leaf  is  hinged  so  that  it  may  be  thrown  back  out  of  the  way  when 
the  work  is  being  removed  or  inserted.  The  rapidity  with  which 
this  can  be  accomplished  is  a  means  of  increasing  the  production. 
Protection  from  Cutting  Tools  and  Sharp  Corners.  In  many 
cases,  standard  machine  tools  are  so  designed  that  the  table  is  well 
away  from  the  cutting  tool  in  its  loading  position.  In  the  case  of 


EQUIPMENT 


133 


drill  jigs,  the  operator  withdraws  them  from  beneath  the  spindle  of 
the  machine  before  he  unloads  them.  Sometimes,  however,  the 
design  of  the  machine  tool  or  fixture  is  such  that  the  fixture  cannot 
be  removed  from  under  the  cutting  tools.  In  such  cases,  if  the  fix- 
ture cannot  be  designed  to  open  on  the  side  away  from  the  cutting 
tool,  provision  should  be  made  for  rolling  or  rocking  the  fixture 
away  from  the  tool.  If  this  cannot  be  done,  some  safety  device 


Machinery 


Fig.  9.     Construction  of  Spline  Milling  Fixture 

should  be  designed  which  will  stop  the  cutting  tool  and  prevent  it 
from  starting  while  the  operator's  hand  is  in  any  position  of  danger. 
Safety  guards  and  automatic  stopping  devices  on  punch  presses 
are  good  examples. 

It  is  evident  that  the  operator  will  handle  a  jig  or  fixture  without 
sharp  edges  much  more  quickly  and  surely  than  one  on  which  he 
is  continually  tearing  his  hands.  It  is  the  standard  practice  of 


134  INTERCHANGEABLE   MANUFACTURING 

most  .tool-rooms  to  remove  all  such  corners  carefully,  whether 
the  drawing  of  the  fixtures  specifies  it  or  not. 

Accessibility  of  Locating  Points.  The  locating  points  in  the 
jigs  and  fixtures  should  be  so  placed  that  they  can  be  readily 
reached.  This  not  only  facilitates  the  cleaning  of  the  fixture 
but  enables  it  to  be  more  accurately  made.  It  also  allows  any 
necessary  corrections  due  to  wear  or  change  in  dimensions  to 
be  quickly  and  economically  made.  These  locating  points 
should  stand  clear  and  be  as  nearly  self-cleaning,  as  regards 
chips  and  dirt,  as  it  is  possible  to  make  them. 


Fig.  10.    Jig  designed  to  permit  Easy  Removal  of  Chips  during 
Various  Operations  on  the  Same  Piece 

Necessity  for  Proper  Chip  Clearances.  Careful  considera- 
tion should  always  be  given  to  the  provision  of  suitable  chip 
clearances.  If  this  point  is  neglected,  the  operator  will  often 
spend  more  time  in  removing  the  chips  from  the  fixture  than 
he  does  on  any  other  operation.  On  the  other  hand,  if  he  does 
not  remove  them,  inaccurate  work  will  result.  Many  ingenious 
devices  have  been  developed  for  cleaning.  A  jet  of  compressed 
air  or  a  stream  of  oil  or  soda  water  properly  directed  often 
accomplishes  this  task  quickly  and  well,  and  this  demands  that 


EQUIPMENT 


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136  INTERCHANGEABLE   MANUFACTURING 

obtaining  chip  clearances.  The  construction  drawing  of  this 
fixture  is  shown  in  Fig.  n.  The  part  to  be  drilled  is  located  in 
one  place  by  the  tapered  tongue  B  shown  in  section  A- A,  and 
is  clamped  by  a  leaf.  The  body  of  the  jig  is  a  skeleton  frame 
which  contains  no  pockets  to  catch  the  chips.  The  leaf  swings 
open,  thus  allowing  all  chips  to  be  easily  brushed  or  blown  out. 
Results  Obtained  with  Proper  Chip  Clearances.  The  follow- 
ing example  shows  the  results  actually  obtained  in  production 
by  attention  to  these  details.  A  certain  firm  contracted  to 
make  200,000  small  pinions  as  shown  in  Fig.  12.  It  will  be 
noted  that  three  of  the  six  teeth  are  partly  removed.  The 


Machinery 


Fig.  12.    Pinion  having  Three  Teeth  partially  removed 

pinions  were  made  from  brass  pinion  stock  being  extruded  to 
form,  after  which  they  were  drilled  and  cut  off  in  an  automatic 
screw  machine,  and  counterbored  in  a  drilling  machine  to  re- 
move the  stock  from  three  of  the  teeth. 

The  drill  jig  first  employed  in  the  last  operation  was  one 
with  a  leaf  in  which  several  pinions  were  held.  It  was  designed 
and  built  without  much  thought,  as  the  job  seemed  simple  and 
unimportant.  The  parts  were  so  small  and  difficult  to  handle 
rapidly  and  the  chips  were  so  troublesome  that  the  best  rate  of 
production  possible  with  this  jig  was  about  160  an  hour.  This 
was  so  far  under  the  estimated  production  that  a  serious  loss 
on  the  contract  seemed  inevitable.  After  a  little  study,  a  new 


EQUIPMENT 


137 


jig  —  incidentally  a  cheaper  one  than  the  original  —  was  de- 
signed and  built.  This  jig  is  shown  in  Fig.  13.  A  pair  of  paral- 
lel jaw  pliers  A  was  used  as  a  basis.  These  had  special  jaws 
B  inserted  to  hold  the  pinions,  one  of  them  carrying  plate  C 
which  had  three  holes  that  served  as  drill  bushings.  Spring 
D  which  was  attached  to  one  jaw,  opened  the  pliers,  while  the 
grip  of  the  operator  closed  them  and  held  the  pinion  in  position. 
A  cast-iron  parallel  E  about  two  inches  high  was  clamped  to 
the  table  of  the  drilling  machine,  and  two  rods  F  which  served 


Machinery 


Fig.   13. 


Jig  developed  for  handling  Pinions  in 
Counterboring  Operation 


as  feet  to  level  the  device,  were  riveted  to  the  handles  of  the 
pliers.  The  bottom  of  the  pinion  rested  on  the  cast-iron  parallel, 
while  the  top  rested  against  the  drill  plate  C. 

The  operator  handled  this  jig  as  follows:  About  half  a  dozen 
pinions  were  placed  on  the  cast-iron  parallel  approximately  an 
inch  apart.  The  open  pliers  were  placed  over  the  first  piece 
and  then  closed.  The  shape  of  the  jaws  insured  that  the  piece 
when  clamped  would  be  in  the  proper  position  for  the  operation. 
The  pliers  were  then  slid  under  the  spindle  of  the  drilling  ma- 
chine and  the  pinion  was  counterbored.  To  unload,  the  operator 


138  INTERCHANGEABLE    MANUFACTURING 

moved  the  jaws  away  from  the  cast-iron  parallel  as  he  released 
his  grip.  The  completed  parts  and  all  chips  then  dropped  out, 
and  he  proceeded  to  pick  up  the  next  pinion.  The  rate  of  pro- 
duction with  this  jig  was  over  500  an  hour,  and  nearly  double 
the  estimated  output. 

Simplicity  and  Standardization  of  Jigs  and  Fixtures.  Need- 
less to  say,  the  greater  the  rigidity  of  the  tools  the  greater  the 
accuracy  of  the  product.  Whenever  possible,  the  pressure  of 
the  cutting  tools  should  be  withstood  by  a  solid  part  of  the 
fixture  and  not  by  a  clamp.  Fixtures  which  are  permanently 
fastened  to  the  machine  should  be  sufficiently  rugged  to  with- 
stand all  use  and  abuse.  Independent  jigs  which  must  be  lifted 
or  turned  over  in  operation  should  be  as  light  as  they  can  safely 
be  made.  The  design  in  all  cases  should  be  as  simple  as  possible 
because  simplicity  is  a  primary  source  of  economy.  This  sim- 
plicity, however,  is  seldom  attained  spontaneously.  It  is  the 
result  of  constant  study  and  careful  and  painstaking  develop- 
ment. It  may  be  safely  asserted  that  designs  which  are  not 
simple  are  incompletely  developed. 

Economy  in  the  construction  of  jigs  and  fixtures  offers  a  field 
for  standardization.  Not  only  the  various  drill  bushings, 
operation  levers,  drill  jig  feet,  vise  jaws,  etc.,  but  also  the  base- 
plates for  jigs  and  fixtures,  various  clamping  devices,  leaves, 
etc.,  can  be  standardized  to  advantage.  This  is  now  common 
practice  in  many  up-to-date  tool-rooms. 

Methods  Employed  in  Manufacturing  Fixtures.  The  in- 
formation required  on  fixture  drawings  depends  to  a  large  extent 
upon  the  methods  to  be  employed  in  the  construction  of  these 
fixtures.  One  of  two  general  methods  is  usually  employed. 
First,  the  complete  fixture  may  be  made  by  one  man  or  a  small 
group  of  men  working  in  close  cooperation.  Second,  the  work 
on  all  fixtures  may  be  resolved  into  its  elements  and  one  man 
or  one  group  of  men  may  do  only  the  work  of  a  certain  type, 
such  as  planing,  milling,  boring,  etc. 

Each  method  has  certain  advantages  and  certain  disadvan- 
tages. Among  the  disadvantages  of  the  first  method  are  the 
following:  When  one  man  performs  all  types  of  operations, 


EQUIPMENT  139 

either  the  tool-room  must  be  over-equipped  with  machinery  or 
a  considerable  amount  of  time  will  be  lost  by  the  various  men 
in  waiting  their  turn  to  use  certain  machines.  Again,  few  men 
are  equally  expert  in  operating  all  types  of  machinery.  The  use 
of  this  method  prevents  the  specialization  of  the  men  on  those 
operations  in  which  they  are  most  expert.  In  order  to  overcome 
the  foregoing  disadvantages,  the  second  method  has  been  used. 
This  often  results  in  reducing  the  amount  of  tool-room  equip- 
ment and  increasing  the  total  output.  On  the  other  hand,  the 
elimination  of  the  disadvantages  of  the  first  method  has  resulted 
in  the  loss  of  many  of  its  advantages.  With  the  first  method, 
the  toolmaker  takes  a  personal  interest  in  the  development  and 
completion  of  a  certain  fixture.  It  is  a  complete  mechanism 
in  which  he  can  take  personal  pride,  and  this  advantage  is  lost 
by  the  adoption  of  the  second  method.  Furthermore,  a  careful 
distinction  must  be  made  between  working  time  and  elapsed 
time.  The  adoption  of  the  second  method,  while  it  often  re- 
duces the  total  working  time  required  to  complete  tools,  inevi- 
tably increases  the  elapsed  time  between  the  receipt  of  the 
order  and  the  delivery  of  the  completed  fixture. 

To  operate  successfully  under  the  second  method,  a  certain 
amount  of  work  must  be  kept  ahead  of  each  operation.  This 
means  that  although  the  machines  and  men  may  be  kept  busy, 
each  piece  of  work  must  wait  its  turn  at  each  operation.  Thus, 
in  many  cases,  the  elapsed  time  required  to  complete  the  fix- 
tures under  the  second  method  is  more  than  double  the  elapsed 
time  required  under  the  first.  For  replacement  work,  which 
can  be  anticipated  and  started  well  in  advance,  and  for  new 
work  when  time  is  not  essential,  the  second  method  is  often  the 
better.  In  other  cases,  if  ultimate  economy  is  desired,  the  first 
method  must  be  employed.  Take  for  example  an  emergency 
repair.  If  we  balance  the  decreased  cost  in  the  tool-room  under 
the  second  method  against  the  increased  cost  of  the  idle  time 
of  productive  machinery  and  labor,  we  shall  find  that  we  have 
lost  more  than  we  have  gained.  The  same  holds  true  for  the 
construction  of  equipment  for  a  new  product  in  a  case  where 
time  is  essential. 


140  INTERCHANGEABLE   MANUFACTURING 

Tolerances  on  Fixture  Drawings.  The  drawings  required  in 
the  tool-room  under  the  first  method  of  production  need  be 
functional  ones  only.  Except  on  important  dimensions  of 
functional  locations,  etc.,  tolerances  will  be  of  doubtful  value. 
Detailed  drawings  of  the  individual  parts  of  fixtures  must  be 
supplied,  yet  tolerances  and  clearances  on  these  details  can  be 
safely  omitted.  On  the  other  hand,  drawings  to  be  used  with 
the  second  method  should  include  tolerances  on  all  but  atmos- 
pheric fits.  The  direction  of  the  tolerances  on  those  surfaces 
which  are  fitted  at  assembly  should  leave  surplus  metal.  Those 
parts  which  assemble  without  fitting  should  have  their  dimen- 
sions and  tolerances  expressed  in  the  same  manner  as  on  the 
component  drawings  of  the  interchangeable  product  itself. 

The  establishment  of  the  tolerances  on  the  functional  locations 
is  identical  in  all  cases.  Variations  in  these  dimensions  will  be 
reproduced  in  the  product  itself.  The  effect  of  these  will  be  to 
reduce  the  manufacturing  tolerance.  It  is  obvious  that  these 
variations  must  be  in  that  direction  which  will  hold  the  product 
within  the  established  tolerances.  Naturally,  then,  these  toler- 
ances should  be  kept  as  small  as  possible.  The  fixtures  are  usually 
made  but  once,  while  the  product  must  be  reproduced  many 
times  over.  The  amount  of  this  tolerance  on  the  jigs  and  fix- 
tures should  be  a  fair  percentage  of  the  component  tolerance 
and  20  per  cent  should  be  sufficient  in  most  cases.  It  is  clear 
that  the  location  of  drill  bushings  for  holes  which  may  vary 
0.020  inch  is  not  so  important  as  for  holes  which  can  vary  but 
0.002  inch. 

It  should  be  kept  in  mind  that  in  many  cases  adjustment  is 
provided  on  the  machine  tools  to  align  the  work-holding  fixture 
correctly  in  relation  to  the  cutting  tool.  Such  is  the  case  on 
milling,  planing,  and  other  similar  machine  tools.  In  the  case 
of  jigs  for  drilling  and  boring,  however,  no  such  adjustment  is 
possible.  Such  equipment  must,  therefore,  be  constructed  with 
greater  accuracy  than  milling  fixtures.  It  is  thus  apparent  that 
the  original  tolerances  and  clearances  for  those  surfaces  ma- 
chined on  this  type  of  equipment  must  be  designated  on  the 
component  drawings  with  great  care.  It  is  obvious  that  if 


EQUIPMENT  141 

needlessly  severe  tolerances  are  required  by  the  component 
drawings,  the  cost  of  the  equipment  will  be  greatly  increased 
and  no  commensurate  benefit  will  be  derived  from  it. 

Checking  and  Testing  Jigs  and  Fixtures.  The  most  effective 
method  of  checking  jigs  and  fixtures  is  to  set  them  up  and  make 
the  required  cuts  on  sample  pieces.  This,  however,  is  not 
always  possible  and  so  model  parts  are  a  good  substitute.  Such 
parts  are  invaluable  for  checking  purposes  during  the  construc- 
tion of  any  of  the  special  manufacturing  equipment.  The  final 
inspection  of  the  completed  fixtures  should  be  a  functional 
inspection  only.  The  operating  parts  must  function  properly 
and  the  functional  locations  must  bear  the  proper  relation  to 
one  another.  The  component  drawings,  as  well  as  the  fixture 
drawing,  should  be  consulted.  Any  fixture  which  will  insure 
the  proper  machining  of  the  component  part  is  satisfactory  as 
regards  its  accuracy.  If  the  fixture  drawing  has  been  carefully 
completed,  the  information  given  there  will  indicate  reasonable 
limits.  If  the  fixture,  however,  exceeds  those  limits,  but  does 
insure  the  completion  of  the  component  part  within  the  estab- 
lished tolerances,  and  without  imposing  over-severe  conditions 
in  the  production,  the  fixture  should  not  be  rejected  because  of 
this  technicality.  It  is  well  to  keep  a  record  of  such  deviations 
for  reference.  The  purpose  of  this  inspection  is  not  to  see  how 
much  fault  can  be  found  with  the  work  accomplished,  but  to 
determine  as  surely  as  possible  whether  or  not  it  will  fulfill  the 
purpose  for  which  it  is  intended.  This  is  never  definitely  de- 
termined until  the  fixture  has  actually  produced  satisfactory 
work. 

Tolerances  to  be  Allowed  on  Cutting  Tools.  In  connection 
with  Figs.  4  and  5,  several  sources  of  variations  in  the  product 
were  mentioned.  Some  of  these  errors  are  fixed  quantities, 
others  are  variable.  In  discussing  the  cutting  tools,  we  will 
consider  two  sources  of  variations  —  the  first  variable,  the 
second  practically  fixed.  The  first  source  of  error,  which  has 
been  previously  mentioned,  is  that  due  to  the  wear  on  the 
cutting  edges  of  the  tool.  This,  as  we  have  seen,  results  in 
leaving  more  metal  on  the  piece.  Disregarding  all  other  factors 


142  INTERCHANGEABLE   MANUFACTURING 

of  error,  and  assuming  that  the  wear  is  equal  on  all  surfaces, 
we  should  make  the  tools  to  the  minimum  metal  sizes  of  the 
component  drawings,  in  order  to  insure  their  longest  life.  When 
faces  are  neither  male  nor  female,  the  initial  size  of  the  tool 
should  be  the  mean  dimension.  But  if  a  tolerance  of,  say,  20 
per  cent  of  the  component  tolerance  has  been  permitted  on  the 
jigs  and  fixtures,  the  cutting  tools  should  be  made  approxi- 
mately the  same  percentage  inside  the  minimum  metal  sizes  of 
the  component,  except  those  made  to  the  mean  dimensions. 
Tools  made  to  such  dimensions  should  produce  work  within 
the  established  limits.  If,  however,  sample  parts  made  by 
such  tools  are  beyond  the  established  limits,  they  will  vary 
outside  of  the  minimum  metal  sizes.  In  such  cases  the  tools 
can  be  salvaged,  as  metal  must  be  removed  from  the  tools  to 
correct  this  fault. 

The  second  source  of  variation  in  the  product  is  initial  error 
in  the  size  or  form  of  the  tool  itself.  Tolerances  for  tools  should 
be  established  from  experience  gained  in  actual  practice  as 
carefully  as  the  tolerances  are  established  for  the  components 
themselves.  Cutting  tools  must  be  replaced  over  and  over 
again  in  the  course  of  production.  On  one  hand,  in  order  to 
reduce  the  first  cost  of  these  tools,  their  tolerances  should  be 
as  liberal  as  possible.  On  the  other  hand,  to  lengthen  their  life 
and  thus  reduce  the  number  of  replacements,  they  should  be 
held  as  close  to  the  minimum  metal  limits  as  other  conditions 
will  permit.  The  economical  balance  between  these  two  factors 
establishes  the  proper  tolerances.  In  practice,  the  fixed  error 
due  to  inaccuracies  in  machine  tools  and  fixtures  can  readily  be 
determined  after  production  is  under  way.  This  error,  properly 
added  to  or  subtracted  from  the  minimum  metal  limit  of  the 
component,  establishes  the  maximum  metal  limit  of  the  cutting 
tool.  This  maximum  metal  limit  should  be  the  basic  dimen- 
sion of  the  cutting  tool.  The  application  of  the  tolerance  then 
establishes  the  minimum  limit  of  new  cutting  tools.  Whenever 
the  established  tolerance  on  the  component  is  exacting,  the 
tool  should  be  made  adjustable  if  possible,  thus  enabling  wider 
limits  to  be  established  on  the  tools,  reducing  their  initial  cost 


EQUIPMENT  143 

and  prolonging  their  life.  Such,  for  example,  is  the  purpose  of 
interlocking  milling  cutters. 

Maintaining  Tolerances  on  Tools  Machining  Several  Surfaces. 
It  is  well  to  note  that  the  more  surfaces  machined  by  a  single 
tool,  the  more  difficult  it  is  to  maintain  close  tolerances,  either 
on  the  tool  itself  or  on  the  product.  Take,  for  instance,  the 
cutting  of  a  thread.  If  a  die  is  used,  the  three  main  elements 
are  carried  on  one  tool;  namely,  the  form,  the  lead,  and  the 
diameter.  Adjustment  for  the  diameter  is  possible,  but  the 
form  and  lead  are  fixed.  Under  present  conditions,  variations 
develop  in  the  lead  and  shape  when  the  tool  is  hardened  and 
these  are  difficult  and  expensive  to  correct.  As  one  die  is  re- 
placed with  another,  these  variations  are  different.  If  the 
tolerances  on  the  component  are  sufficiently  liberal,  the  use 
of  dies  is  satisfactory.  If  they  are  severe,  satisfactory  dies 
are  very  expensive,  although  the  direct  production  costs  are 
low.  In  these  cases,  if  the  threads  are  milled  or  chased,  the  form 
alone  is  carried  by  the  tool.  Such  a  tool  is  readily  and  accu- 
rately made  and  maintained.  The  diameter  is  controlled  by  the 
setting  of  the  tool,  and  is  readily  adjusted  and  readjusted.  The 
lead  is  controlled  by  the  lead-screw  of  the  machine  and  is  prac- 
tically constant.  Lead-screws  can  be  obtained  within  any 
reasonable  degree  of  accuracy. 

Sufficient  chip  clearances  must  be  provided  on  all  cutting  tools 
if  a  free  cutting  tool  is  to  be  obtained.  The  proper  rate  of  the 
cutting  edge  should  be  determined  as  early  as  possible  and 
carefully  maintained.  The  design  should  be  as  simple  and 
rugged  as  conditions  will  permit.  The  individual  design  and 
requirements  of  the  cutting  tools,  of  course,  depend  on  the 
nature  of  the  material  to  be  machined  and  the  methods  of 
machining  employed.  The  drawings  of  the  cutting  tools  should 
be  made  with  as  much  care  and  in  conformity  with  the  same 
basic  principles  as  the  component  drawings  themselves.  When 
continuous  production  is  involved,  it  is  necessary  to  provide  a 
constant  supply  of  cutting  tools.  This  supply  of  cutting  tools 
must  be  available  for  instant  use  with  as  little  adjusting  or 
correction  as  possible. 


144 


INTERCHANGEABLE   MANUFACTURING 


O 

60 


EQUIPMENT 


145 


Special  Equipment  for  Machining  Automobile  Transmission 
Cases.  In  order  to  make  clear  the  application  of  some  of  the 
principles  stated,  examples  of  properly  designed  jigs  and  fix- 
tures will  now  be  presented.  For  the  first  example,  part  of  the 
special  equipment  required  to  manufacture  the  transmission 
case  shown  in  Figs.  14  and  15  will  be  considered.  As  over  forty 
operations  in  all  are  required  to  machine  this  case,  space  will 


SECTION    A-A,    FIG.   1 


Machinery 


Fig.  15.     Sectional  View  of  Transmission  Case 

not  permit  a  detailed  discussion  of  each,  but  as  many  are  either 
practically  duplicate  or  are  handled  in  well-known  conventional 
ways,  only  the  most  instructive  operations  will  be  dealt  with. 
The  drawings  shown  in  Figs.  14  and  15  are  not  complete,  many 
dimensions  and  projections  which  do  not  affect  the  operations 
to  be  discussed  having  been  purposely  omitted.  The  first  opera- 
tion is  to  snag  and  chip  the  casting.  This  is  a  bench  job  and 
requires  no  special  equipment.  The  second  operation  is  to  drill 


i46 


INTERCHANGEABLE   MANUFACTURING 


the  main  bearing  hole  A,  Fig.  15,  as  shown  in  the  operation 
drawing,  Fig.  16. 

Operation  drawings  of  this  sort  are  of  great  value  to  the 
tool  designer  and  also  to  the  shop  foreman  and  machine  opera- 
tor. Only  the  information  required  for  a  single  operation 
appears  on  each  drawing,  thus  making  it  handy  for  reference  in 
the  shop  and  also  preventing  any  possibility  of  using  a  wrong 
dimension.  They  may  be  drawn  to  a  much  smaller  scale  than 
the  component  drawings  and  still  contain  information  that 
cannot  always  be  placed  on  the  component  drawing  without 
danger  of  misuse.  In  practice,  they  are  readily  made.  Small 
drawings  of  each  projection  of  the  work  are  made,  and  then  the 


LOCATING 

P 


^^  EQUALIZING 
/     CLAMPS 


(F)CLAMP 
LOCATING 


'-II 


Mashinery 


Fig.  16.    Operation  Drawing  for  drilling  Main  Bearing  Hole  in 
Small  End 

section  or  projection  required  for  any  particular  operation  draw- 
ing is  traced  and  the  required  dimensions  and  notes  added.  Free- 
hand tracings  are  often  sufficient.  No  great  amount  of  detail 
need  be  shown  in  these  drawings.  All  that  is  required  is  enough 
to  indicate  the  machined  surface  in  question,  the  required  regis- 
ter or  locating  surfaces,  and  the  clamping  points.  On  opera- 
tions where  little  or  no  special  equipment  is  required,  no  opera- 
tion drawings  are  necessary.  When  these  operation  drawings 
are  developed  in  conjunction  with  the  operation  lists  described 
earlier  in  this  chapter,  a  further  advantage  is  gained.  The  work 
of  designing  the  tools  and  fixtures  can  be  readily  and  safely 
distributed  among  several  designers,  either  in  the  same  organi- 


EQUIPMENT  147 

zation  or  in  independent  shops.  When  time  is  essential  in 
commencing  production,  this  factor  becomes  of  great  importance. 
Jigs  for  Drilling  Holes  in  Transmission  Case.  The  first 
machining  operation  is  important  in  several  ways.  As  the 
surface  machined  at  this  time  becomes  the  register  point  for 
many  of  the  succeeding  operations,  it  is  necessary  that  the 
casting  be  carefully  centralized  in  the  jig,  which  is  shown  in 
Figs.  17  and  18.  A  study  of  these  illustrations  and  the  opera- 
tion drawing  shown  in  Fig.  16,  will  make  clear  the  general  con- 


Fig.  17.    Front  View  of  Jig  for  drilling  Main  Bearing  Hole  in  Small  End 

struction  of  the  jig.    The  locating  points  and  clamps  are  lettered 
alike  on  these  three  illustrations. 

The  operation  of  this  jig  is  as  follows:  Leaf  E,  Fig.  17,  being 
open,  the  transmission  case  is  slid  along  the  locating  rails  H 
until  the  shifter  housing  face  B,  Fig.  15,  comes  in  contact  with 
bar  A,  Fig.  17,  and  the  locating  lugs  come  in  contact  with  the 
buttons  B.  This  locates  the  case  in  one  plane  and  also  squares 
it  up.  Clamp-screw  C  operates  on  an  angle  against  the  fillet 
on  boss  C,  Fig.  15,  and  holds  the  case  down  on  rails  H  and  also 


148 


INTERCHANGEABLE    MANUFACTURING 


against  bar  A  (Fig.  17).  Handwheel  D  (Fig.  18)  operates  the 
equalizing  clamps  G  (Fig.  17)  which  both  locate  and  clamp 
the  case  in  another  plane.  The  leaf  E  is  then  closed  and  the 
clamp-screw  F  which  it  carries  is  used  to  complete  the  rigid 
clamping  of  the  case.  Thus,  the  case  is  located  and  clamped 
in  three  planes.  As  this  transmission  case  is  a  relatively  large 
piece,  the  design  of  jigs  and  fixtures  to  clamp  it  with  only  one  or 
two  motions  of  the  operator's  hands  would  be  a  complicated  and 
difficult  task  unless  some  type  of  hydraulic  or  pneumatic  clamp- 


Fig.  18     Rear  View  of  Jig,  showing  Mechanism  for  operating 
Equalizing  Clamps. 

ing  mechanism  were  employed.  This  has  been  successfully 
accomplished  in  a  simple  and  effective  manner,  but  is  not  yet 
common  practice.  An  example  of  such  a  jig  will  be  presented 
later. 

A  drawing  of  the  jig  just  described  is  shown  in  Fig.  19.  It 
will  be  seen  that  stop  A  is  pivoted  to  allow  for  inequalities  in 
the  casting.  The  equalizing  clamps  G  are  operated  by  levers 
which  are  actuated  by  nuts,  one  threaded  right-hand,  and  the 


EQUIPMENT 


149 


other  left-hand.  These  nuts  are  operated  by  means  of  similar 
threads  on  the  handwheel  spindle.  It  will  be  noted  that  this  jig 
is  rugged  and  simple,  that  all  functional  locations  and  parts  are 
accessible,  and  also  that  the  chip  clearances  are  liberal,  thus  making 


Machinery 


Fig.  19.     Assembly  Drawing  of  Jig  illustrated  in  Figs.  17  and  18 

a  fixture  that  is  readily  cleaned  and  operated.     It  is  also  one  that 
requires  little  attention  in  service. 

The  third  operation  consists  of  drilling  and  rough-counterboring 
the  main  bearing  hole  D,  Fig.  15,  and  facing  and  turning  flange  E 
and  boss  F.  This  is  done  in  a  large  Porter  &  Johnston  lathe.  An 
attachment  to  the  spindle  of  the  machine  which  runs  in  a  cat- 
head at  its  outer  end,  locates  the  case  on  an  arbor  through  the  hole 


INTERCHANGEABLE    MANUFACTURING 


drilled  in  the  preceding  operation  and  from  the  back  of  the 
flange.  The  case  is  also  aligned  and  clamped  around  the  out- 
side of  the  flange.  Two  locating  lugs  cast  on  the  bell  of  the  case 
assist  in  locating  and  driving  the  work.  These  two  locating 
lugs  are  removed  after  the  machining  is  completed .  The  face  of 
flange  E  and  the  center  line  of  the  main  bearing  holes  A  and 


Machinery 


Fig.  20. 


Operation  Drawing  for  counterboring  the  Main  Bearing 
Hole  in  the  Small  End  and  facing  the  Boss 


D  now  become  the  primary  locating  points  for  most  of  the  suc- 
ceeding operations. 

Jigs  and  Tools  Used  in  Various  Operations.  The  fourth 
operation  is  performed  on  a  boring  mill  and  consists  of  facing 
the  case  to  length  and  rough-counterboring  hole  A,  Fig.  15. 
The  operation  drawing  for  this  operation  is  shown  in  Fig.  20. 
The  case  is  located  in  the  jig  shown  in  Fig.  21,  by  flange  E  and 
boss  F,  Fig.  15,  and  also  on  an  arbor  which  passes  through  the 


EQUIPMENT 


main  bearing  hole  D.  This  arbor  is  hollow  to  receive  the  pilot 
on  the  boring-bar,  thus  helping  to  align  it.  The  case  is  squared 
up  by  two  studs  A,  Fig.  21,  which  bear  on  the  shifter  housing 
face  B,  Fig.  15,  and  is  supported  centrally  and  clamped  by  two 


Fig.  21 


Jig  and  Boring-bar   used   in  machining  Case   to  Length  and  counter- 
boring  Main  Bearing  Hole  in  Small  End 


clamp-screws  B,  Fig.  21.    The  bearing  in  the  end  of  the  jig  is 
large  enough  to  permit  the  boring-bar  C  to  enter  with  the  cut- 
ting tools  assembled. 
The  boring-bar  for  this  operation  is  shown  in  Fig.  22.    Pilot 


Fig.  22.     Boring-bar  disassembled  from  the  Jig  illustrated  in  Fig.  21 

A  enters  the  hollow  arbor  on  the  jig.  Surface  B  carries  a  ream- 
ing tool  which  corrects  the  alignment  of  hole  A,  Fig.  15.  Slot 
C  carries  a  combined  boring  and  facing  tool  which  faces  surface 


152  INTERCHANGEABLE   MANUFACTURING 

C,  Fig.  15,  and  so  machines  the  case  to  length,  and  rough- 
counterbores  hole  A,  Fig.  15.  This  makes  a  self -registering 
tool  for  the  depth  of  the  counterbore.  It  will  be  noted  that 
this  depth  of  counterbore  is  given  in  a  different  way  on  the 
operation  drawing  from  the  way  it  is  shown  on  the  component 
drawing.  This  is  only  a  roughing  operation,  and  it  will  be  noted 
that  stock  is  left  for  finishing.  A  later  operation  will  bring  the 
dimensions  for  this  surface  in  accordance  with  the  component 
drawing.  Surface  D  on  the  tool,  Fig.  22,  runs  in  the  large  bear- 
ing in  the  fixture,  while  the  lock-nut  E  is  adjustable  and  acts 
as  a  stop  for  regulating  the  position  of  the  facing  tool.  A  little 


Fig.  23.    Fixture  employed  in  reaming  both  Main  Bearing  Holes 
of  the  Transmission  Case 

study  will  show  that  this  arrangement  will  maintain  the  length 
of  the  case  in  accordance  with  the  component  drawing.  The 
face  of  the  flange  is  located  against  fixed  points  on  the  fixture. 
The  face  of  the  shoulder  of  the  large  bushing  D  is  held  in  a  fixed 
position  in  relation  to  the  locating  blocks  on  the  fixture.  There- 
fore, it  makes  a  reliable  registering  point  for  a  tool  which  must 
maintain  a  specified  relation  to  these  locating  blocks. 

The  fifth  operation  is  to  mill  the  shifter  housing  face  B,  Fig. 
15,  and  the  clutch  hand-hole  face  G.  This  is  a  straight  milling 
operation  performed  on  a  large  planer-type  milling  machine 
which  machines  a  large  number  of  castings  at  one  time.  The 


EQUIPMENT 


153 


sixth  operation  consists  of  hand-reaming  the  counterbores  of 
holes  A  and  D,  Fig.  15,  both  for  diameter  and  depth.  The 
stand  shown  in  Fig.  23  is  provided  to  hold  the  case  and  to  pilot 
the  tools  while  hand- reaming.  The  case  is  placed  on  the  arbor 
shown  at  the  left  to  ream  counterbore  A,  Fig.  15,  with  the  face  of 
flange  E  resting  on  the  locating  blocks.  The  pin  A,  Fig.  23, 
enters  a  hollow  lug  at  the  large  end  of  the  case  to  prevent  it 


Mnchinerv 


Fig.  24.     Operation  Drawing  showing  Methods  of  locating  and 
clamping  Work  for  Seventh  Operation 

from  rotating.  The  end  of  the  arbor  acts  as  a  stop  for  the  ream- 
ing tool,  thus  maintaining  the  conditions  called  for  on  the  com- 
ponent drawing.  The  case  is  then  inverted  and  placed  on  the 
arbor  shown  at  the  right  in  Fig.  23  to  ream  counterbore  D, 
Fig.  15.  The  block  B  rests  against  the  shifter  housing  face  B, 
Fig.  15,  to  prevent  rotation  of  the  case.  The  bottom  of  the 
counterbore  finished  on  the  other  arbor  rests  on  shoulder  C, 


154 


INTERCHANGEABLE   MANUFACTURING 


while  the  end  of  the  arbor  acts  as  a  stop  for  the  reamer,  thus 
maintaining  the  location  of  the  bottom  of  this  counterbore  in 
accordance  with  the  information  on  the  component  drawing. 

Operations  on  Vertical  Milling  and  Profiling  Machines.  The 
seventh  operation  consists  of  milling  surfaces  H  and  7,  Fig.  15, 
and  the  corresponding  surfaces  around  the  idler  shaft  hole  M , 
Fig.  14.  This  operation  is  performed  on  a  vertical  milling 
machine  equipped  with  an  auxiliary  cutter-head  to  reach  down 
into  the  case.  The  operation  drawing  for  this  job  is  shown  in 


Fig.  25.    Fixture  used  in  Milling  Operation  on  Case,  and  Parts 
for  supporting  Auxiliary  Cutter-head 

Fig.  24.  The  fixture  used  for  this  operation  is  shown  at  the  left 
in  Fig.  25.  The  case  is  located  in  one  plane  on  the  flange,  squared 
up  and  located  in  the  second  plane  on  the  shifter  cover  face  B, 
Fig.  15,  and  located  in  the  third  plane  by  being  centralized  on 
the  body.  The  center  line  of  holes  A  and  D,  Fig.  15,  is  not  used 
as  a  locating  point  for  this  operation,  for  two  reasons:  First, 
the  requirements  of  the  surface  milled  in  this  operation  are 
that  they  be  parallel  to  the  flange  and  that  they  be  maintained 
at  the  specified  distance  from  the  flange  and  from  each  other. 
Therefore,  the  location  of  the  case  in  relation  to  the  main  bearing 
holes  is  unimportant.  The  second  reason  is  that  the  use  of  an 
arbor  in  this  fixture  would  greatly  increase  the  amount  of  time 
required  to  load  and  unload  the  work.  The  case  is  clamped  on 


EQUIPMENT  155 

surface  /,  Fig.   15,  by  the  leaf  and  on  surface  C  by  the  clamp- 
screw. 

The  construction  of  the  auxiliary  cutter-head  is  shown  in  Fig.  26. 
The  driving  spindle  E  fits  into  the  spindle  of  the  vertical  milling 
machine  and  drives  the  cutters  F  and  G  through  a  train  of  gears. 


Fig.  26.     Auxiliary  Cutting  Head  used  in  milling  Surfaces  on  Inside  of  Case 

The  drawing  of  this  cutter-head  should  be  self-explanatory.  The 
cutter-head  is  supported  by  a  bridge  composed  of  pieces  A,  B, 
and  C,  shown  at  the  right,  Fig.  25,  which  is  clamped  to  the  milling 
machine,  as  shown  in  Fig.  27.  Support  B  is  clamped  at  the  proper 
height  on  the  dovetail  of  the  machine  column.  Support  A  is  clamped 
to  the  dovetail  on  the  knee  of  the  machine,  while  bridge  C  is  sup- 


156 


INTERCHANGEABLE   MANUFACTURING 


ported  on  the  tops  of  the  two  supports.  The  auxiliary  cutter- 
head  is  then  fastened  on  surfaces  D.  Vertical  adjustment  is 
provided  for  the  outer  end  of  the  bridge  which  rests  on  A, 
while  none  is  required  at  the  other  end. 

The  knee  of  the  milling  machine  is  adjustable  up  and  down 
to  control  the  position  of  the  cuts,  and  as  support  B  holds  a 
fixed  position  in  relation  to  the  machine  while  A  moves  with 
the  knee,  it  is  necessary  to  provide  the  adjustment  for  the  outer 


Machinery 


Fig.  27.     Assembled  View  of  Parts  clamped  on  Milling  Machine  to 
support  the  Auxiliary  Cutting  Head 

end  at  A.  The  fixture,  Fig.  25,  is  attached  to  the  table  and  fed, 
with  the  table,  to  the  cutters.  Attention  is  called  to  the  fact 
that  the  loading  side  of  the  fixture  is  on  the  side  farthest  from 
the  cutters.  This  arrangement  enables  the  feed  of  the  table  to 
be  kept  to  a  minimum. 

The  eighth  operation  consists  of  milling  the  outside  boss 
around  the  idler  shaft  hole  M,  Fig.  14,  on  the  small  end  of  the 
case.  Ordinarily,  this  operation  would  be  performed  on  a  verti- 
cal milling  machine,  or  with  a  facing  tool  on  a  drilling  machine. 
In  this  particular  case,  however,  profiling  machines  were  avail- 
able, while  vertical  milling  machines  were  not.  Furthermore, 
this  cut  is  a  relatively  light  one,  which  a  profiling  machine  can 
handle  satisfactorily,  and  so  this  type  of  machine  is  used.  This 


EQUIPMENT  157 

indicates  to  a  certain  degree  how  the  design  of  the  equipment 
is  determined  by  the  machine  tools  available.  The  fixture  for 
this  operation  is  shown  to  the  right  in  Fig.  28.  The  case  is 
located  on  the  face  of  flange  E,  Fig.  15,  and  by  boss  F,  and  is 
clamped  on  the  back  of  the  flange.  This  is  an  instance  where  a 
pneumatic  or  hydraulic  clamping  device  could  be  used  to  great 
advantage  and  save  fully  half  the  setting-up  time. 

The  fixture  not  only  consists  of  a  work-holding  device,  but 
also  acts  as  the  stand  for  the  working  gage.  Lug  A  is  used  for 
registering  the  position  of  the  cutters  and  also  for  registering 
the  flat  step-gage  D  which  is  used  to  test  the  finished  height  of 
the  boss.  Because  the  maximum  distance  between  the  table 


Fig.  28.     Equipment  provided  to  adapt  a  Profiling  Machine  for  a 
Milling  Operation  on  the  Case 

of  the  profiling  machine  and  its  cutter-spindles  was  not  great 
enough,  raising  blocks  C  were  provided  to  lift  the  heads  of  the 
machine  sufficiently  to  permit  the  transmission  case  to  be 
machined.  Bracket  B  was  also  required  to  support  the  end  of 
the  operating  handle  shaft  in  its  raised  position. 

Drilling,  Reaming,  and  Milling  Operations.  The  ninth  and 
tenth  operations,  respectively,  consist  of  drilling  and  reaming 
the  countershaft  holes  K  and  Z,,  Fig.  15.  Both  of  these  opera- 
tions are  performed  on  horizontal  radial  drilling  machines,  and 
the  work-holding  fixtures  employed  in  each  case  are  practically 
identical  in  design.  The  jig  for  the  tenth  operation  is  shown 
in  Fig.  29.  The  case  is  located  on  the  face  of  flange  E,  Fig.  15, 


158  INTERCHANGEABLE   MANUFACTURING 

and  on  arbors  through  the  main  bearing  holes  A  and  D,  and  is 
squared  up  by  two  equalizing  plungers  which  bear  on  surface 
B.  The  case  is  clamped  by  the  end  of  the  hollow  arbor  B, 
Fig.  29,  which  is  drawn  back  against  the  bottom  of  the  counter- 
bore  A,  Fig.  15.  Arbor  A,  Fig.  29,  which  passes  through  B 
contains  a  groove  which  receives  leaf  F.  Thus,  when  arbor  A 
is  drawn  up  by  the  clamping  nut  C,  this  leaf  is  drawn  up  against 
the  end  of  arbor  B,  clamping  the  case  between  the  face  of  the 
flange  and  the  bottom  of  the  counterbore  previously  referred  to. 
When  leaf  F  is  swung  aside,  arbor  A  is  withdrawn.  Bracket  D 
is  provided  to  support  the  end  of  the  arbor  A  in  its  loading 
position.  Handles  G  and  H  are  used  to  wring  the  arbors  into 


Fig.  29.     Work-holding  Fixture  for  Drilling  and  Reaming  Opera- 
tions on  Radial  Drilling  Machine 

the  case,  as  these  arbors  are  a  very  close  fit  in  the  counterbores 
which  are  held  to  a  tolerance  of  o.ooi  inch.  Handle  E  operates 
the  equalizing  plungers  which  bear  against  surface  B,  Fig.  15, 
and  square  up  the  case.  The  eleventh  operation  consists  of 
drilling  and  reaming  the  idler  shaft  hole  M,  Fig.  14.  A  very 
similar  jig  is  used  in  this  operation,  the  principal  difference 
being  that  the  case  is  squared  up  by  a  plug  in  countershaft  hole 
K,  Fig.  15,  instead  of  by  equalizing  plungers  acting  against 
surface  B. 

The  twelfth  and  thirteenth  operations,  respectively,  con- 
sist of  milling  the  surfaces  of  the  tire  pump  face  N  and  pedal 
support  boss  0,  Fig.  14.  Both  of  these  operations  are  performed 


EQUIPMENT  159 

on  a  profiling  machine,  and  the  same  fixture  is  used.  This 
fixture  is  shown  in  Fig.  30.  The  case  is  located  on  holes  A  and 
D,  Fig.  15,  by  arbor  A,  Fig.  30,  and  hollow  plug  B,  and  on 
surface  C,  Fig.  15,  by  shoulder  C.  It  is  squared  up  by  the  pin 
D  projecting  into  hole  K,  Fig.  15.  Two  screws  E  are  adjusted 
to  suit  surface  B,  Fig.  15,  to  support  the  case  against  the  clamp- 
ing device  F,  Fig.  30.  The  spring  plunger  G  is  locked  by  the 
clamp-screw  H  and  supports  the  case  against  the  thrust  of  the 
cutters.  Arbor  A  has  a  bayonet  cam- slot  which  engages  a  stud 
in  the  hollow  plug  B,  clamping  the  case  between  the  bottom 


Fig.  30.     Another  Fixture  provided  for  a  Milling  Operation  on  a 
Profiling  Machine 

of  counterbore  D  and  surface  C,  Fig.  15.  Stud  K  is  the  register 
point  for  the  tool  when  milling  the  pedal  support  boss,  while 
stud  L  is  the  register  point  for  the  tool  when  milling  the  tire 
pump  face.  Due  to  the  position  of  this  fixture  on  the  machine, 
all  of  the  operating  handles  must  be  on  the  front  or  on  the  sides. 
This  is  the  reason  for  the  extensions  to  the  two  clamps  which 
are  carried  through  the  side  of  the  fixture. 

In  the  fourteenth  operation,  the  clutch  shaft  hole  P,  Figs. 
14  and  15,  is  drilled  and  reamed.  This  operation  is  performed 
on  a  boring  mill.  The  jig,  with  slip  bushings  for  the  reamers, 


l6o  INTERCHANGEABLE    MANUFACTURING 


Fig.  31.     Jig  employed  in  Drilling  and  Reaming  Operations  on 
a  Boring  Mill 


Fig.  32.     Simple  Stand  provided  for  Supporting  Case    during 
Operations  on  Drain  Plug  Hole 


EQUIPMENT  l6l 

is  shown  in  Fig.  31.  The  case  is  located  by  the  usual  register 
points,  on  the  main  bearing  bore  and  counterbore  D,  Fig.  15, 
and  on  the  flange  E.  It  is  squared  up  by  the  countershaft  hole 
L,  and  clamped  from  the  small  end  against  the  face  of  the  flange. 
A  groove  at  the  end  of  arbor  A,  Fig.  31,  receives  leaf  B.  Clamp- 
nut  C  draws  the  arbor  back,  thus  clamping  the  case  between 
surface  C,  Fig.  15,  and  the  face  of  the  flange.  Leaf  B  is  swung 
aside  when  unloading,  thus  permitting  arbor  A  to  be  withdrawn. 


Fig.  33.     Drill  Jig  used  for  drilling  Seven  Small  Holes  in  Trans- 
mission Case  at  One  Time 

Bracket  G  supports  the  shoulder  of  the  large  arbor  in  its  with- 
drawn position.  Handle  D  is  provided  to  assist  in  wringing  the 
large  arbor  into  the  case.  Lug  F  enters  the  case  through  the 
opening  in  the  shifter  housing  face  B,  Fig.  15.  Plug  E,  Fig.  31, 
wrings  into  the  countershaft  hole  L  and  into  the  lug  F  to  align 
the  case. 

The  fifteenth  operation  is  performed  on  a  drilling  machine, 
and  consists  of  drilling,  tapping,  and  counterboring  drain  plug 


l62 


INTERCHANGEABLE   MANUFACTURING 


hole  /,  Fig.  15,  and  facing  the  surface  of  its  bore.  The  exact 
location  of  this  drain  plug  hole  is  of  no  importance.  A  conical 
spot  is  cast  in  the  boss  which  is  used  to  locate  and  center  the 
drill  point.  The  simple  stand  shown  in  Fig.  32,  unprovided 
with  bushings  or  clamps,  supports  the  case  on  the  drilling  ma- 
chine table.  Further  elaboration  of  this  simple  design  would 
not  improve  its  effectiveness  in  any  way.  The  drilling  machine 
is  fitted  with  a  quick-change  collet  to  permit  the  operator  to 
change  the  drill,  tap  and  facing  tools  rapidly. 

Jigs  for  Drilling  a  Number  of  Holes  at  One  Time.    The  six- 
teenth operation  on  the  transmission  case  consists  of  drilling 


Fig.  34.    Cradle  Jig  and  Cluster  Plate  employed  in  drilling  Twelve 
Holes  on  a  Multiple-spindle  Drilling  Machine 

seven  small  holes  Q  (see  Fig.  14)  in  the  small  end  of  the  case. 
This  operation  is  performed  on  a  single-spindle  drilling  machine 
equipped  with  a  special  multiple  drill  head.  The  jig  used  is 
illustrated  in  Fig.  33.  The  face  of  flange  E,  Fig.  15,  rests  on 
rails  B,  while  the  large  stud  C  enters  the  main  bearing  counter- 
bore  A,  Fig.  15,  and  the  small  stud  D  enters  the  idler  shaft  hole 
M,  Fig.  14,  thus  centering  and  squaring  the  case.  The  operat- 
ing handle  E  is  pulled  forward  and  down  as  indicated  by  the 
arrow,  thus  lowering  the  head  by  means  of  the  crank  and  con- 


EQUIPMENT  163 

necting-rod  A,  and  clamping  the  case  during  the  drilling  opera- 
tion. The  operator  holds  handle  E  down  with  one  hand  while 
he  operates  the  drilling  machine  with  the  other.  Attention  is 
called  to  the  commendable  features  of  this  jig,  which  include 
simplicity  of  design,  accessibility  of  all  functional  surfaces,  chip 
clearances,  and  rapid  operation. 

The  seventeenth  operation  is  performed  on  a  multiple-spindle 
drilling  machine  provided  with  a  lifting  table,  and  consists  of 
drilling  eight  holes  R,  Fig.  14,  and  four  holes  around  hole  D, 
Fig.  15.  The  jig  used  for  this  operation,  together  with  the 
cluster  plate  for  guiding  the  drills,  is  shown  in  Fig.  34.  These 
parts  are  shown  assembled  in  Fig.  35.  The  jig  is  in  the  form 
of  a  cradle  to  permit  it  to  be  tipped  for  removing  and  inserting 
the  product.  Otherwise  the  table  of  the  machine  would  have 
to  be  lowered  an  excessive  amount  to  accomplish  the  same 
result. 

The  case  is  located  by  an  arbor  through  the  main  bearing 
holes  A  and  D,  Fig.  15,  and  rests  on  surface  C,  Fig.  15.  The 
jig  is  rocked  to  its  drilling  position  and  locked  there  by  the 
spring  plunger  A,  Fig.  35.  Four  spring  plungers  B  rest  against 
the  back  of  the  flange,  and  are  locked  in  position  by  clamp-screws. 
These  support  the  flange  against  the  thrust  of  the  drills.  The 
case  is  located  radially  by  stud  E  which  enters  countershaft 
hole  K,  Fig.  15.  The  cluster  plate  slides  on  the  column  of  the 
drilling  machine,  and  is  held  down  by  the  springs  on  the  three 
supporting  rods  F,  while  the  washers  at  the  end  of  these  rods 
limit  its  downward  travel.  As  the  table  of  the  machine  is 
raised,  the  arbor  in  the  jig  enters  hole  C  in  the  cluster  plate, 
Fig.  34,  thus  aligning  the  case,  while  blocks  D  press  against  the 
face  of  the  flange  of  the  transmission  case,  clamping  it  in  posi- 
tion. This  makes  a  simple  and  quickly  operated  jig.  As  the 
table  is  raised  toward  the  drilling  position,  the  work  is  clamped, 
and  as  the  table  is  lowered,  the  work  is  automatically  undamped. 
This  automatic  clamping  feature  effects  a  considerable  saving 
in  time  when  setting  up  work. 

The  eighteenth  operation  consists  of  drilling  six  holes  in  sur- 
face B,  Fig.  15,  and  two  holes  in  bosses  S,  Fig.  14.  This  opera- 


164 


INTERCHANGEABLE   MANUFACTURING 


I 


* 


EQUIPMENT 


165 


Fig.  36.     Another  Drill  Jig  for  drilling  a  Number  of  Holes  at  One 
Time  on  a  Single-spindle  Drilling  Machine  having  a  Special  Head 


Fig.  37, 


Jig  used  for  drilling  and  Reaming  Operations  performed 
on  Boring  Mill 


tion  is  performed  on  a  single-spindle  drilling  machine  equipped 
with  a  special  multiple-spindle  drill  head.  The  jig  is  of  the  box 
type,  as  shown  in  Fig.  36.  The  case  is  located  on  an  arbor 


i66 


INTERCHANGEABLE   MANUFACTURING 


through  the  main  bearing  holes  A  and  D,  Fig.  15,  radially  by  a 
stud  in  countershaft  hole  K,  and  is  clamped  from  the  bottom 
of  counterbore  D  against  surface  C,  Fig.  15.  The  hollow  plug 
A,  Fig.  36,  fits  over  the  arbor  into  the  main  bearing  counterbore 
D,  Fig.  15.  Leaf  B  is  then  swung  under  the  washer  and  bears 
against  the  end  of  the  hollow  plug  as  clamp  C  is  tightened.  This 
leaf  has  the  same  function  as  a  slotted  washer  and  is  employed 
in  place  of  such  a  washer  to  eliminate  an  additional  loose  piece 
on  the  jig. 

The  nineteenth  and  twentieth  operations  are  similar  drill- 
ing operations  on  small  holes,  performed  on  single-spindle  drill- 


Fig.  38.    Drill  Jig  provided  for  drilling  Three  Small  Holes  in  Pedal 
Support  Boss 

ing  machines  equipped  with  special  multiple-spindle  drill  heads. 
The  transmission  case  is  held  in  simple  jigs  for  both  of  these 
operations.  The  twenty-first  operation  consists  of  drilling  and 
reaming  hole  T,  Fig.  14,  and  is  performed  on  a  boring  mill. 
The  fixture  for  this  operation  is  shown  in  Fig.  37.  The  holding 
points  and  methods  of  clamping  are  identical  to  those  of  the 
fixture  shown  in  Fig.  31.  The  twenty-second  operation  con- 
sists of  drilling  holes  U,  Fig.  14,  on  a  single-spindle  drilling 


EQUIPMENT  167 

machine  provided  with  a  special  multiple-spindle  drill  head. 
The  jig  shown  in  Fig.  38  is  similar  to  the  one  shown  in  Fig.  33. 
The  transmission  case  is  located  from  the  main  bearing  holes 
A  and  Z>,  Fig.  15,  by  arbor  A  and  hollow  plug  B.  Surface  V, 
Fig.  15,  is  held  against  shoulder  C  by  the  shoulder  of  the  hollow 
plug  acting  against  face  C,  Fig.  15.  The  plug  is  held  by  a  stud 
which  engages  the  bayonet  cam-slot  in  arbor  A.  The  case  is 
located  radially  by  stud  D  which  enters  countershaft  hole  L, 
Fig.  15.  The  operating  handle  E  is  moved  as  indicated  by  the 
arrow,  which  lowers  the  head  F  carrying  the  drill  bushings  and 
also  a  stud  which  enters  hole  T,  Fig.  14. 


Fig.  39.    Equipment  employed  in  performing  Various  Operations 
on  Filler  Hole 

Operations  on  Filler  Hole.  The  twenty- third  operation  con- 
sists of  drilling,  chamfering,  spot-facing,  and  tapping  filler  hole 
W,  Fig.  14.  This  operation  is  performed  on  a  single-spindle 
drilling  machine  by  means  of  the  jig  shown  in  Fig.  39.  The  case 
is  located  on  an  arbor  through  the  main  bearing  holes  A  and 
D,  Fig.  15,  and  clamped  against  surface  C,  Fig.  15,  in  the  usual 
manner.  It  is  located  radially  by  a  stud  which  enters  the 
countershaft  hole  K,  Fig.  15.  The  spring  plunger  A,  which  is 
locked  by  the  clamp-screw  B,  supports  the  under  side  of  the 
filler  boss  against  the  thrust  of  the  cutting  tools.  The  drilling 


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INTERCHANGEABLE   MANUFACTURING 


Fig.  40.     Fixture  equipped  with  Trunnioned  Roller  Bearings,  used 
in  tapping  Holes 


Fig.  41.    Pivoted  Ball-bearing  Fixture  used  in  Tapping  Operation 


EQUIPMENT 


169 


machine  is  equipped  with  a  quick-change  collet  attachment  to 
promote  the  speedy  change  of  tools. 

The  next  six  operations  are  all  minor  drilling  operations 
requiring  simple  jigs.  The  thirtieth  operation  consists  of  coun- 
tersinking all  the  holes  to  be  tapped  in  succeeding  operations 
with  a  small  electric  hand  drill,  equipped  with  the  proper  coun- 
tersinks. No  special  work-holding  device  is  required.  The 
thirty-first  and  thirty-second  operations  are  tapping  operations 
performed  on  a  tapping  machine,  without  the  use  of  special 


Machinery 


Fig.  42. 


Drawing  of  Tapping  Fixture  illustrated  in  Fig.  40,  equipped  with 
Trunnioned  Roller  Bearings 


fixtures.  The  eight  remaining  machining  operations  are  tapping 
operations  for  which  fixtures  are  provided.  Some  of  them  are 
simple  stands,  while  others  are  mounted  on  ball  or  roller  bear- 
ings to  make  lighter  work  for  the  operator  in  shifting  the  jig 
from  position  to  position.  Two  of  these  jigs  will  be  described. 
Jigs  Provided  for  Tapping  Operations.  The  thirty-fourth 
operation  consists  of  tapping  the  six  holes  previously  drilled  in 
face  B,  Fig.  15,  and  is  performed  on  a  tapping  machine.  The 
fixture  is  shown  in  Fig.  40.  The  transmission  case  is  located 
on  an  arbor  through  the  main  bearing  holes  A  and  Z),  Fig.  15, 


170 


INTERCHANGEABLE    MANUFACTURING 


and  is  squared  up  by  a  stud  which  enters  the  countershaft  hole 
K,  Fig.  15.  The  fixture  is  mounted  on  two  sets  of  roller  bear- 
ings which  permit  it  to  be  readily  rolled  in  two  directions.  The 
construction  is  shown  in  Fig.  42.  In  order  to  make  the  travel 
of  the  rolls  as  short  as  possible,  they  are  made  with  two  small 
trunnions  as  shown  at  A.  These  trunnions  roll  on  the  bottom 
rails,  while  the  large  periphery  of  the  rolls  is  in  contact  with 
the  upper  rails.  The  effect  of  this  construction  is  that  the 
upper  part  of  the  fixture  can  move  about  six  inches  while  the 


Machinery 


Fig.  43.     Pivoted  Ball-bearing  Fixture  shown  in  Fig.  41 

rolls  have  moved  only  about  one  inch.  The  lower  rails  are 
shorter  than  the  upper  ones  in  the  lower  sections  of  the  fixture. 
The  thirty-seventh  operation  consists  of  tapping  holes  U, 
Fig.  14,  and  is  performed  on  a  tapping  machine  by  means  of 
the  fixture  shown  in  Fig.  41.  The  case  is  supported  on  an  arbor 
through  the  main  bearing  hole  D,  Fig.  15,  and  is  clamped  and 
squared  up  against  the  face  of  flange  E,  Fig.  15,  by  a  slotted 
washer  which  bears  against  face  X,  Fig.  15.  This  washer  is 
drawn  back  by  a  clamp-nut  on  the  plunger  through  the  arbor. 
The  case  is  located  radially  by  the  stud  seen  in  Fig.  41,  which 
enters  countershaft  hole  L,  Fig.  15.  The  upper  part  of  the 


EQUIPMENT  171 

fixture  is  pivoted  and  revolves  on  a  large  ball  race,  as  shown  in 

Fig.  43- 

The  foregoing  descriptions  of  some  of  the  special  manufac- 
turing equipment  for  an  automobile  transmission  case  have 
not  been  given  for  the  purpose  of  illustrating  how  such  a  part 
may  be  machined,  but  rather  to  indicate  in  some  degree  the 
many  factors  which  must  be  considered  in  the  design  of  any 
special  manufacturing  equipment.  These  descriptions  are  in- 
complete, yet  a  careful  study  of  the  component  drawing  and 
the  illustrations  will  supply  many  of  the  omissions  in  the  de- 
scriptions. With  few  exceptions,  no  mention  has  been  made 
of  the  cutting  tools,  as  these  have  been  for  the  most  part  stand- 
ard mills,  boring-bars,  drills,  taps,  reamers,  or  similar  standard 
tools. 

Attention  is  called  to  the  general  grouping  of  the  opera- 
tions. First,  the  boring  and  facing,  next  the  milling,  then  the 
drilling  and  reaming  of  the  larger  supplementary  holes,  fol- 
lowed by  a  few  profiling  operations.  The  final  operations 
consist  of  drilling  and  tapping  the  many  smaller  holes.  For 
large  production,  the  machine  tools  required  would  usually  be 
so  grouped  that  the  parts  would  pass  consecutively  from 
one  operation  to  the  next.  For  smaller  rates  of  production 
where  the  machine  tools  are  not  kept  set  up  constanly  for  one 
operation,  they  are  often  grouped  together  by  types  of  ma- 
chines. In  either  case,  the  foregoing  general  arrangement  of 
operations  would  be  satisfactory.  A  few  minor  changes  in 
sequence  might  be  made  to  advantage,  but  the  general  lay-out 
would  remain  unchanged.  In  order  to  cover  some  points  which 
the  foregoing  examples  have  not  touched  upon,  a  few  examples 
of  tools  and  several  additional  fixtures  will  be  presented.  These 
will  indicate  possibilities  in  design,  which  effect  savings  in 
direct  labor  costs. 

Pneumatic  Clamping  Devices.  It  has  been  pointed  out  that 
the  use  of  pneumatic  or  hydraulic  clamping  devices  is  necessary 
on  many  fixtures  for  large  parts,  if  the  clamping  is  to  be  per- 
formed by  a  single  movement  or  two  of  the  operator.  The  best 
known  examples  of  pneumatic  clamping  devices  are  air  chucks. 


172 


INTERCHANGEABLE    MANUFACTURING 


Fig.  44  shows  a  drill  jig,  together  with  the  part  that  is  to  be 
drilled;  this  is  clamped  by  air  pressure.  A  flexible  hose,  at- 
tached to  the  air  line,  is  connected  with  this  fixture  by  part  A . 
The  part  to  be  drilled  is  slid  into  the  jig  from  the  right,  with 
the  machined  face  down.  The  opening  of  a  valve  operates  four 
pistons,  one  built  into  leaf  B,  one  in  leaf  C,  and  the  others  on 
the  back  of  the  fixture  which  operates  clamps  D  and  E.  These 
hold  the  part  firmly  in  position  while  the  holes  are  being  drilled. 
Sight-holes  are  provided  so  that  any  mislocation  of  the  part  is 


Fig.  44.     Drill  Jig  equipped  with  Air-operated  Clamping  Devices 

readily  detected.  As  a  further  safeguard  against  movement  of 
the  work,  the  large  hole  F  is  first  drilled,  and  a  plug  is  then 
inserted  in  it  through  the  bushing.  This  jig  is  easily  and 
rapidly  operated,  and  the  design  of  the  clamping  pistons  is  as 
simple  as  that  of  any  other  clamping  device  would  be. 

There  are  ocasions  when  a  convenient  air  line  is  not  present, 
or  where  the  attachment  of  a  flexible  hose  would  interfere  with 


EQUIPMENT 


o  S 


£•*  ~? 


a  I  g-l 


3  £ 


174 


INTERCHANGEABLE   MANUFACTURING 


end  in  the  illustration  and  B  at  the  left.  This  bar  carries  four 
tool -holders  C,  D,  E,  and  F,  and  is  used  to  face  the  bosses  on 
the  spindle  head  of  a  machine  tool.  The  head  to  be  machined 


Fig.  46.    Close-up  View  of  One  of  the  Facing  Bars  in  Fig.  45,  show- 
ing Grooves  used  in  feeding  Cutter  across  the  Surface 


Fig.  47.    Typical  Example  of  Fixtures  used  in  Continuous  Milling 
Operations  for  producing  Parts  in  Large  Quantities 

is  clamped  in  position  on  the  table  of  a  boring  mill  with  the 
facing  bar  in  position,  and  the  bearing  caps  are  then  assem- 
bled. Bracket  G  is  also  clamped  to  the  table  of  the  machine 


EQUIPMENT  175 

to  prevent  endwise  movement  of  the  outer  spindle.  As  the 
facing  bar  revolves,  the  inner  bar  is  fed  in,  which  feeds  the 
facing  tools  carried  in  the  tool-holders  into  the  faces  of  the 
bosses.  This  is  accomplished  by  means  of  angular  tongues  (such 
as  may  be  seen  at  end  B  on  the  inner  bar)  sliding  in  angular 
grooves  (such  as  shown  at  A,  Fig.  46),  which  are  cut  on  the 
tool-holders.  The  outer  spindle  is  relieved  to  permit  the  facing 
tools  to  pass  beyond  the  inside  edges  of  the  faces  of  the  bosses. 
The  tongues  and  grooves  are  so  cut  that  when  one  tool  has 


Fig.  48.     Continuous  Drilling  Fixture  having  Three  Work-holding  Stations 

traveled  its  full  distance,  it  stops  close  to  this  relief  until  all 
the  other  tools  have  finished  cutting.  The  cutting  edge  of  the 
tool  has  then  traveled  beyond  the  face  of  the  boss  so  that  it 
does  not  score  the  face  by  dwelling  there.  After  all  the  tools 
have  completed  their  cuts,  the  work  is  removed  from  the  ma- 
chine and  the  facing  tools  returned  to  their  starting  position  by 
withdrawing  the  inner  bar.  The  tools  are  adjustable,  which  en- 
ables the  lengths  and  relative  positions  of  the  bearings  to  be 
accurately  maintained.  With  this  tool,  several  bosses  can  be 
faced  in  little  more  time  than  one. 

Milling  and  Drilling  Fixtures.  Fixtures  and  machines  for 
continuous  milling  operations  are  chiefly  used  where  the  produc- 
tion of  any  large  quantity  of  parts  is  required.  An  example  of 
such  a  milling  fixture  is  shown  in  the  illustration  Fig.  47.  When 
these  fixtures  are  used,  the  machine  is  working  constantly  in- 


176  INTERCHANGEABLE   MANUFACTURING 

stead  of  being  idle  during  the  time  the  operator  is  engaged  in 
removing  the  finished  parts  and  inserting  new  work  in  place  in 
the  fixture. 

While  continuous  milling  fixtures  are  quite  common,  similar 
drilling  fixtures  are  not  so  well  known.  Fig.  48  shows  such  a 
fixture  designed  to  be  used  on  a  single-spindle  drilling  machine; 
the  fixture  is  mounted  on  the  machine  in  a  vertical  position. 
The  special  multiple-spindle  drill  head  A  is  driven  by  the  spindle 
of  the  machine  and  also  raised  and  lowered  in  the  usual  manner 
by  the  machine  spindle.  The  bars  B  guide  the  head  and  also 
prevent  it  from  rotating.  The  two  brackets  C  and  D  are  bolted 
to  the  column  of  the  machine,  the  bracket  D  containing  bushings 
to  assist  in  the  alignment  of  the  drills.  The  special  table  E  is 
also  bolted  to  the  column  of  the  drilling  machine,  in  place  of 
the  standard  table.  The  jig  is  pivoted  and  supported  by  the 
bracket  D  and  table  E.  This  jig  contains  three  work-holding 
stations,  so  that  while  drilling  one  piece,  the  operator  removes 
and  inserts  parts  in  the  other  stations,  thus  obtaining  a  much 
higher  production  rate. 

All  the  foregoing  examples  of  special  manufacturing  equip- 
ment are  used  for  relatively  large  parts,  but  the  same  general 
principles  apply  equally  to  small  pieces.  Certain  detailed 
practices  which  are  sometimes  followed  for  one  class  of  work 
are  not  always  feasible  when  the  size  of  the  parts  becomes  much 
larger  or  smaller.  The  fundamental  problems,  however,  are 
the  same,  and  economical  solutions  require  the  careful  use  of 
the  same  basic  factors. 


CHAPTER  IX 
GAGES   IN   INTERCHANGEABLE  MANUFACTURING 

A  GAGE  is  an  instrument  or  apparatus  for  measuring  a  specific 
dimension.  Every  manufactured  part  is  measured  during  its 
production  and  after  its  completion,  in  one  way  or  another.  This 
applies  equally  to  a  single  piece  made  for  a  special  machine  or  as 
a  repair  part,  or  to  a  hundred  thousand  duplicate  parts  manu- 
fuctured  for  an  interchangeable  product.  The  mere  removing 
or  shaping  of  the  raw  material  in  itself  is  seldom  a  difficult  or 
exacting  task.  The  critical  point  is  in  stopping  this  process  at 
the  proper  moment.  The  approach  to  this  point  can  be  watched 
only  by  some  form  of  measurement.  The  most  elementary 
method  of  measuring  a  part  is  to  try  it  with  the  companion 
parts  with  which  it  is  to  operate.  Such  was  common  practice 
in  the  early  stages  of  mechanical  industry.  This  practice  neces- 
sarily continues  to  a  great  extent  with  repair  work  and  also  in 
the  construction  of  small  numbers  of  special  machines,  jigs,  and 
fixtures. 

A  later  method  consisted  of  measuring  the  parts  individually, 
with  standard  measuring  tools  such  as  scales,  calipers,  verniers, 
and  micrometers.  In  many  cases,  these  measurements  were 
merely  preliminary  to  the  fitting  together  of  the  parts  at  as- 
sembly. Fitting  at  assembly  is  expensive.  It  takes  time  and 
requires  a  relatively  large  amount  of  space  and  highly  skilled 
labor.  Most  of  the  metal  removed  at  this  time  is  done  by  hand. 
If  any  great  amount  of  metal  is  to  be  removed,  the  part  must 
be  taken  back  to  the  machine  shop  and  relocated  on  the  machine, 
thus  interrupting  other  work.  Under  such  conditions,  the  eco- 
nomic production  of  any  great  quantities  of  duplicate  mechanisms 
is  impossible. 

Gages  an  Economical  Necessity.  Interchangeable  manu- 
facturing was  developed  primarily  to  eliminate  these  conditions. 
If  parts  could  be  made  close  enough  to  some  uniform  size  so  that 

177 


178  INTERCHANGEABLE   MANUFACTURING 

most,  at  least,  of  this  fitting  could  be  eliminated,  it  was  evident 
that  larger  production  could  be  secured  with  the  expenditure 
of  the  same  effort.  Furthermore,  many  parts  could  be  machined 
in  advance  and  carried  in  stock,  thus  making  earlier  deliveries 
possible  in  many  cases.  Clearly,  one  of  the  most  essential  factors 
of  such  a  plan  is  a  reliable  means  of  measuring  each  part  as  it 
is  made.  This  measuring,  to  be  effective,  must  insure  uniformity 
and  be  economical.  The  use  of  standard  measuring  instruments 
such  as  micrometers  is  not  always  reliable  in  measuring  large 
numbers  of  duplicate  parts.  In  the  first  place,  for  many  ex- 
acting conditions,  measurements  hurriedly  made  by  several  dif- 
ferent men  do  not  prove  sufficiently  uniform.  In  the  second 
place,  many  of  the  surfaces  to  be  measured  are  not  readily  ac- 
cessible by  standard  measuring  tools.  And,  in  the  third  place, 
while  both  of  the  preceding  conditions  may  often  be  satis- 
factorily met,  the  time  consumed  by  this  method  would  be  too 
great  to  be  economical.  In  order  to  meet  all  these  conditions, 
special  measuring  tools  known  as  gages  have  been  developed. 

Gages  are  an  integral  part  of  interchangeable  manufacturing 
equipment.  They  comprise  that  part  of  the  equipment  the 
purpose  of  which  is  to  measure  the  product,  as  distinguished 
from  that  part  of  the  equipment  the  purpose  of  which  is  to  change 
the  form  of  the  material  or  to  hold  the  part  during  a  manufac- 
turing operation.  Under  this  broad  definition  of  a  gage,  it  is  ap- 
parent that  some  of  the  manufacturing  equipment  may  be  not 
only  a  holding  device,  but  also  a  gage.  In  fact  it  is  good  practice 
to  make  fixtures  so  that  an  unserviceable  part  cannot  be  inserted. 
It  often  happens  that  when  the  normal  manufacturing  variations 
of  certain  machining  processes  are  small  and  within  known 
limits,  a  gage  may  be  employed  to  test  the  size  or  form  of  the 
cutting  tool,  and  not  be  applied  directly  to  the  product.  At 
other  times,  a  gage  in  the  form  of  a  setting  block  for  the  position 
of  the  cutting  tool  is  made  as  an  integral  part  of  the  fixture. 
Therefore,  to  determine  the  character  of  the  gages  that  are  re- 
quired for  the  production  of  any  particular  part,  it  is  necessary 
to  consider  both  the  requirements  of  the  part  in  question  and  the 
other  manufacturing  equipment  that  is  provided. 


GAGES  179 

Classification  of  Gages  According  to  Use.  In  general,  there 
are  three  purposes  for  which  gages  may  be  needed:  First,  in  the 
manufacture  of  large  numbers  of  duplicate  pieces,  it  is  a  measure 
of  economy  to  detect  and  discard  all  unserviceable  parts  as  soon 
as  possible,  thus  saving  the  expenditure  of  additional  effort  on 
such  parts.  The  gages  provided  for  the  purpose  are  commonly 
known  as  working  gages.  These  are  often  limit  gages,  placed 
in  the  hands  of  the  machine  operator  to  check  each  individual 
machining  operation  as  it  is  performed. 

Second,  it  is  necessary  to  check  the  parts  as  they  are  trans- 
ferred from  one  manufacturing  department  to  another,  and  also 
before  the  finished  parts  are  sent  to  the  stock-room  or  assembling 
floor,  so  as  to  prevent  unserviceable  parts  from  proceeding 
farther.  It  is  also  customary  to  inspect  the  parts  in  process 
after  certain  groups  of  operations  have  been  performed.  The 
gages  used  for  these  purposes  are  commonly  known  as  inspection 
gages.  Some  of  these  are  limit  gages  which  are  often  duplicates 
of  some  of  the  working  gages,  while  others  are  functional  gages 
which  check  the  results  of  several  operations  at  one  time.  These 
inspection  gages  are  generally  used  by  a  force  of  inspectors  who 
are  independent  of  the  production  department. 

Third,  when  gages  are  used  to  any  extent,  it  is  necessary  to 
have  reliable  standards  as  a  means  of  checking  the  working  and 
inspection  gages  and  to  establish  the  sizes  of  new  gages  as  the 
old  ones  wear  out.  Such  standards  are  variously  known  as 
checks,  reference  gages,  standards,  master  gages,  and  model 
parts.  These  gages  are  usually  kept  in  the  tool-room  or  in  the 
hands  of  a  gage  inspector,  and  their  purpose  is  to  test  the  working 
and  inspection  gages  so  as  to  insure  a  suitable  degree  of  preci- 
sion in  them. 

Required  Accuracy  of  Gages.  Extreme  refinement  in  gages 
is  expensive  and  is  unwarranted  by  the  functioning  of  the  ma- 
jority of  component  parts.  The  accuracy  required  by  a  gage 
depends  in  a  great  measure  upon  the  extent  of  the  manufacturing 
tolerances.  If  these  tolerances  have  been  properly  established, 
only  a  small  percentage  of  them  will  be  exacting.  It  is  evident 
that  a  gage  used  to  measure  a  dimension  which  has  a  tolerance 


i8o 


INTERCHANGEABLE   MANUFACTURING 


of  0.002  inch  must  be  made  closer  to  size  than  one  which  meas- 
ures a  dimension  having  a  tolerance  of  0.020  inch.  It  is  common 
practice  to  establish  the  tolerance  on  a  gage  at  10  per  cent  of 
the  component  tolerance.  A  tolerance  of  less  than  0.0002  inch 
should  seldom  be  specified  unless  the  conditions  are  unusually 
exacting  and  economy  is  no  object. 

With  variations  in  gages,  no  matter  how  slight,  and  with 
parts  passing  through  successive  inspections,  many  misunder- 
standings are  inevitable  unless  precautions  are  taken  to  guard 
against  them.  The  most  common  method  of  meeting  this  con- 


10  PER  CENT  OF 


Machinery 


Fig.   1.     Diagrammatic  Illustration  showing  Differences  between 

Working  and  Inspection  Gages  and  Tolerances  on  these 

Gages 

dition  is  to  establish  the  limits  of  the  working  gages  inside  the 
limits  of  the  inspection  gages.  Fig.  i  is  a  diagrammatical  il- 
lustration showing  the  differences  between  working  and  inspec- 
tion gages  and  the  tolerance  on  these  gages.  The  full  lines 
represent  the  maximum  size  of  a  part  while  the  dotted  lines 
represent  the  minimum  size.  The  maximum  size  of  the  "go" 
inspection  gage  is  identical  with  the  maximum  size  of  the  com- 
ponent. The  minimum  size  of  the  "go"  inspection  gage  is  10 
per  cent  of  the  component  tolerance  smaller  than  its  maximum 
size.  The  maximum  size  of  the  "go"  working  gage  is  identical 


GAGES  l8l 

with  the  minimum  size  of  the  "go"  inspection  gage,  while  its 
minimum  size  is  10  per  cent  of  the  component  tolerance  smaller. 
In  a  similar  manner,  the  minimum  size  of  the  "not  go"  inspec- 
tion gage  is  identical  with  the  minimum  size  of  the  component 
while  its  maximum  size  is  10  per  cent  of  the  component  tol- 
erance larger.  The  minimum  size  of  the  "not  go"  working  gage 
is  identical  with  the  maximum  size  of  the  "not  go"  inspection 
gage,  while  its  maximum  size  is  10  per  cent  of  the  component 
tolerance  larger.  As  the  tolerance  on  the  component  increases,  it 
is  often  advisable  to  reduce  this  percentage.  Thus,  for  plain  plug 
and  snap  gages  a  tolerance  greater  than  from  o.ooi  to  0.002  inch 
is  seldom  necessary. 

Relation  between  Working  and  Inspection  Gages.  It  is  evi- 
dent that  if  the  sizes  of  the  working  gages  are  always  kept  inside 
of  the  sizes  of  the  inspection  gages,  few  questions  should  arise 
due  to  parts  passing  the  working  gages  and  being  rejected  by  the 
inspection  gages.  This  arrangement  may  be  secured  by  making 
and  maintaining  the  gages  as  outlined  in  the  foregoing  or  by  a 
process  of  selection  and  grading.  If  all  the  gages  used  at  the 
same  time  for  the  same  surface  are  checked  concurrently,  those 
permitting  the  widest  variation  in  the  product  should  be  used 
as  inspection  gages,  while  the  others  should  be  used  as  working 
gages.  In  all  cases  the  nominal  sizes  of  the  inspection  limit 
gages  should  be  identical  with  the  limits  of  the  component,  and 
all  tolerances  should  keep  them  within  the  limits  of  the  com- 
ponent. Thus,  the  maximum  gage  may  be  smaller  than  its 
nominal  size  but  never  larger,  while  the  minimum  gage  may  be 
larger  but  never  smaller. 

Such  a  practice  brings  up  two  age-old  arguments:  First,  that 
a  i-inch  plug  will  not  enter  a  i-inch  hole,  and  second,  that  the 
tolerances  on  the  gages  rob  the  manufacturer  of  some  of  the 
tolerances  given  on  the  drawing.  The  answer  to  these  argu- 
ments depends  upon  the  interpretation  of  the  drawing.  If  this 
interpretation  is  that  the  dimensions  and  tolerances  given  on 
the  drawing  represent  the  extreme  sizes  of  the  limit  gages,  and 
all  variations  of  whatever  source  must  come  within  these  limits, 
neither  of  the  above  arguments  has  any  weight;  and  this  is  the 


182  INTERCHANGEABLE   MANUFACTURING 

only  logical  interpretation  that  can  be  used  definitely  and  con- 
sistently. With  this  interpretation,  it  does  not  matter  whether 
the  hole  is  ever  exactly  one  inch  or  not.  As  for  the  second 
argument,  if  the  shop  does  not  attempt  to  maintain  its  product 
within  slightly  smaller  tolerances  than  the  extreme  tolerances, 
too  large  a  percentage  of  parts  will  inevitably  run  outside  of  the 
tolerances  and  be  rejected. 

Gage  Requirements  Controlled  by  Ultimate  Economy.  A 
limit  gage  is  one  that  measures  both  the  maximum  and  mini- 
mum sizes  of  the  component.  Such  gages  usually  check  ele- 
mentary surfaces,  although  they  are  at  times  provided  for  check- 
ing profiles  and  other  composite  surfaces.  The  most  common 
types  of  limit  gages  are  snap  gages,  plug  gages,  ring  gages,  depth 
gages,  and  length  gages.  A  functional  gage  is  one  that  checks 
primarily  the  functional  operation  of  a  component  without 
strict  adherence  to  its  exact  physical  dimensions.  Several  types 
of  these  gages  were  discussed  in  Chapter  V.  The  purpose  of 
such  a  gage  is  to  insure,  as  far  as  possible,  the  proper  assembling 
and  operation  of  all  parts. 

The  extent  to  which  gages  should  be  employed  depends  on 
the  product  and  the  rate  of  production.  If  the  rate  of  produc- 
tion is  low,  it  is  often  possible  to  control  the  accuracy  of  the 
product  with  standard  measuring  instruments.  As  it  increases, 
the  time  spent  in  using  standard  instruments  reaches  a  point 
where  the  time  saved  by  the  use  of  gages  more  than  pays  for  their 
cost.  Gages  should  be  provided  for  only  those  surfaces  which 
it  is  essential  to  maintain  within  certain  dimensions.  Each 
gage  should  have  its  definite  purpose  just  as  any  other  piece  of 
manufacturing  equipment  has  some  definite  duty  to  perform. 
A  gage  is  a  preventive  and  not  a  cure.  Gages  are  required 
wherever  their  use  will  tend  to  prevent  the  production  of  faulty 
work.  Thus  a  more  complete  system  of  gages  is  necessary  in  a 
shop  that  employs  a  large  percentage  of  semi-skilled  labor  than 
in  a  shop  employing  highly  skilled  operatives. 

Interchangeability  between  Parts  Made  in  Different  Shops. 
Experience  has  shown  it  to  be  difficult  to  obtain  interchange- 
able parts  from  several  independent  plants  producing  a  common 


GAGES  183 

product  unless  great  precaution  is  taken  at  the  outset  to  insure 
this  result.  Under  these  conditions  the  most  certain  method  is 
to  maintain  identical  working  and  gaging  points  at  the  various 
plants  for  all  functional  surfaces.  Component  drawings,  properly 
dimensioned,  assist  greatly  in  accomplishing  this  end.  This 
does  not  necessarily  mean  that  the  design  of  the  gages  must 
be  identical.  The  exact  design  of  a  gage  is  never  in  itself  a  matter 
of  great  importance.  The  effectiveness  and  economy  of  the  re- 
sults obtained  are  the  important  considerations.  Usually  the 
gaging  equipment  must  be  very  complete  to  meet  successfully 
the  requirements  of  interchangeability  between  independent 
plants. 

For  the  further  discussion  of  gages,  they  will  be  classified  ac- 
cording to  their  type,  such  as  snap  gages,  ring  gages,  plug  gages, 
profile  gages,  thread  gages,  flush-pin  gages,  functional  gages, 
etc. 

Snap  Gages.  Gages  were  first  developed  as  part  of  the  equip- 
ment necessary  for  manufacturing  large  numbers  of  duplicate 
parts.  Now  gages  are  used  to  a  large  extent  in  the  production 
of  smaller  numbers  of  parts.  In  this  case,  however,  many  modi- 
fications in  the  design,  such  as  adjustable  features,  have  been 
developed  to  keep  the  cost  within  reasonable  limits.  Snap  gages 
for  use  in  the  manufacture  of  large  numbers  of  interchangeable 
parts  will  be  discussed  first.  The  earliest  form  of  snap  gage 
was  the  "one-size"  type;  that  is,  a  gage  to  measure  one  flat 
dimension  only.  This  type  is  still  used  to  a  large  extent  in  tool- 
rooms and  machine  shops  when  limits  are  not  expressed  on  the 
drawings  and  when  the  clearances  for  the  different  fits  are  left 
to  the  judgment  of  the  workmen. 

The  limit  gage  with  two  steps  was  later  developed,  one  step 
being  provided  for  measuring  the  maximum  limit,  and  the  other 
for  measuring  the  minimum  limit.  For  small  parts  produced 
in  large  quantities  the  non-adjustable  gages  are  most  satisfactory. 
Formerly  a  number  of  gage  slots  were  cut  in  one  piece  of  steel  to 
permit  a  combination  of  gages  in  one  piece,  but  the  disadvantage 
of  this  design  was  that  when  one  gage  became  worn  the  whole 
gage  was  lost.  One  method  of  overcoming  this  disadvantage  is 


184 


INTERCHANGEABLE   MANUFACTURING 


to  have  a  filler  block  inserted  on  one  side  of  the  gage  jaw  which 
can  be  replaced  when  the  gage  becomes  worn.  Sometimes  a  com- 
bination of  gages  is  mounted  on  a  ring  similar  to  a  key  ring.  In 
a  later  snap  gage  construction,  individual  gages  are  assembled  in 
convenient  units  and  held  together  by  clamping  strips  and  screws. 

This  construction  permits  the 
easy  removal  of  a  gage  when 
necessary. 

Various  Other  Types  of 
Snap  Gages.  One  type  of 
snap  gage  has  an  intermediate 
step  between  the  two  limit 
steps,  to  aid  the  machine 
operator  in  setting  up  and 
adjusting  the  tools.  In  set- 
ting up  a  machine  for  repeti- 
tion work,  the  object  is  to  set 
the  tools  so  as  to  have  the 
maximum  time  between  ad- 
justments. When  a  circular 
part  is  machined  with  a  form 
tool  or  a  box-tool,  the  piece 
becomes  larger  as  the  tool 
wears.  Therefore,  the  initial 
setting  should  be  as  near  the 

Fig.  2.     Snap  Gage  with  Shallow  .    .  ,,    ,.     ., 

Throat  for  measuring  Lengths  minimum    or       not  gO       limit 

as  other  conditions  will  per- 
mit. The  intermediate  steps  on  these  working  snap  gages  is 
made  to  approximately  the  mean  dimension.  Thus,  if  the 
operator  sets  the  machine  to  produce  work  between  the  mini- 
mum limit  of  the  gage  and  the  intermediate  step,  the  life  of  the 
tool,  as  regards  wear  at  the  particular  setting,  is  equal  to  at  least 
half  of  the  working  tolerance.  These  intermediate  steps  are  not 
used  on  inspection  gages,  as  they  would  serve  no  purpose  there. 
There  are  two  general  types  of  snap  gages,  those  with  deep 
throats  for  measuring  diameters  and  those  with  shallow  throats 
as  illustrated  in  Fig  2,  for  measuring  lengths.  When  the  gage 


GAGES  185 

slot  is  very  narrow,  snap  gages  are  frequently  made  with  a  re- 
movable strip  serving  as  one  side  of  the  gage  slot.  This  con- 
struction permits  the  gaging  surfaces  to  be  readily  ground. 

For  larger  pieces  and  for  smaller  rates  of  production,  adjustable 
snap  gages  have  been  developed.  Gages  of  this  type  are  shown 
in  Fig.  3.  In  common  with  other  types  of  adjustable  tools, 
these  should  be  adjustable  in  the  tool-room  and  fixed  in  the 
manufacturing  departments.  This  result  is  obtained  by  pro- 
viding a  place  for  a  seal  which  must  be  broken  before  the  gage 


Fig;  3.     Adjustable  Snap  Gages  employed  for  Comparatively  Large  Pieces  or 
when  the  Rate  of  Production  is  Small 

can  be  adjusted,  thus  preventing  the  adjustment  from  being 
tampered  with.  These  gages  may  be  readily  adjusted  in  the  tool- 
room to  any  desired  limits,  so  that  a  few  sets  of  them  provide  a 
flexible  and  economical  equipment  of  gages  for  checking  ele- 
mentary dimensions,  such  as  external  diameters,  thicknesses, 
and  lengths. 

Ring  Gages.  Under  some  conditions,  the  use  of  a  snap  gage 
for  testing  diameters  is  not  sufficient,  and  in  such  cases '  ring 
gages  are  employed.  Wherever  possible,  however,  snap  gages 


i86 


INTERCHANGEABLE   MANUFACTURING 


should  be  employed,  as  they  are  more  economical  to  use.  A 
snap  gage  can  be  used  more  rapidly  than  a  ring  gage.  Further- 
more, on  many  parts,  a  machine  operator  cannot  use  a  ring 
gage  without  removing  the  work  from  the  machine.  The  ex- 
tent of  the  tolerance  required  to  manufacture  a  part  economically 
depends  in  a  large  measure  on  the  type  of  gage  employed.  For 
example,  if  a  ring  gage  is  used  in  place  of  a  snap  gage,  any  de- 
parture from  rotundity  or  from  size  affects  the  acceptance  of  the 
part  by  the  gage.  Thus,  in  effect,  a  snap  gage  checks  an  ele- 
mentary surface  while  a  ring  gage  checks  a  composite  one. 


Fig.  4. 


A  Ring  Gage  of  the  Ordinary 
Type 


Fig.  5.     Counterbore 
Plug  Gage 


The  severest  possible  inspection  of  a  cylindrical  surface  is 
obtained  by  the  use  of  "go"  ring  gages  and  "not  go"  snap  gages. 
It  is  therefore  evident  that  a  description  of  the  gaging  and  in- 
spection methods  is  essential  in  the  specifications  to  avoid  mis- 
understandings. The  larger  ring  gages  are  made  as  individual 
gages,  as  shown  in  Fig.  4,  and  are  sometimes  provided  with 
handles.  Several  small  ring  gages  are  often  inserted  into  a  soft 
holder  which  keeps  them  together. 

Plug  Gages.  Plain  plug  gages  are  old  and  simple  forms. 
Standard  plug  gages,  as  with  standard  snap  gages,  are  largely 


GAGES 


I87 


used  in  tool-rooms  and  general  machine  shops.  A  "not  go" 
gage  was  a  later  development  and  is  attached  either  to  the  other 
end  of  the  same  handle  as  the  "go"  gage  or  is  a  separate  gage. 
It  is  common  practice  to  make  standard  handles  and  standard 
plug  gage  blanks,  later  finishing  these  blanks  to  the  size  required 
and  assembling  them  into  the  standard  handles.  Solid  double- 
ended  plug  gages  are  open  to  the  same  objections  as  solid  com- 
bined snap  gages.  If  one  end  becomes  unserviceable,  the  whole 
gage  must  often  be  discarded.  In  standard  limit  plug  gages 
the  minimum  or  "go"  ends  are  made  longer  than  the  maximum 


Fig.  6. 


Two-step  Plug  Gages  used  for  the  Inspection  of 
Through  Holes 


or  "not  go"  ends;  this  practice  is  followed  in  order  to  make  the 
"go"  end  readily  distinguishable  from  the  "not  go"  end,  and, 
furthermore,  as  the  "not  go"  end  is  subject  to  little  wear,  there 
is  no  necessity  for  making  it  very  long. 

When  a  through  hole  is  to  be  gaged,  it  is  customary  to  make 
a  two-step  plug  gage  such  as  shown  in  Fig.  6.  This  permits 
rapid  inspection,  but  the  gage  cost  is  greater  than  when  two 
separate  ends  are  used.  Often,  however,  the  saving  in  inspec- 
tion costs  will  greatly  exceed  the  additional  expense  of  this  type 
of  gage,  so  that  the  practice  is  economical  in  the  long  run. 


1 88 


INTERCHANGEABLE   MANUFACTURING 


The  Pratt  &  Whitney  Co.  manufactures  a  gage  known  as  the 
"star"  gage,  which  is  of  the  expansion  type,  having  four  movable 
measuring  ends.  This  gage  is  used  for  measuring  the  bores  of 
tubes  and  jackets  for  large  guns,  the  bores  of  engine  cylinders, 
etc.  Plug  gages  made  from  flat  stock  are  often  used  to  measure 
the  width  or  length  of  slots  or  grooves.  These  gages  are  fre- 
quently rounded  at  the  end  and  used  for  measuring  the  length 
of  a  splined  slot. 

Plug  Gages  for  Several  Surfaces  and  Taper  Surfaces.  Thus 
far  only  gages  for  elementary  surfaces  have  been  considered. 


Fig.  7.     Taper^lug  Gage 


mKsSSSSSmm 


Fig.  8.     Combination  Plug  and  Snap  Gage 

The  dimensions  for  such  gages  are  readily  determined  from  the 
limits  expressed  on  the  component  drawings.  To  test  concen- 
tricity, however,  the  assembly  requirements  of  the  mating  parts 
must  be  considered.  A  gage  of  this  type  for  testing  the  con- 
centricity of  the  bore  and  counterbore  of  the  main  bearing  of  an 
automobile  transmission  case  is  shown  in  Fig.  5.  Plug  gages  are 
often  made  with  steps  on  the  end  to  gage  both  the  diameter 
and  the  depth  of  a  hole  at  the  same  time.  At  other  times  a 
sliding  collar  is  provided  which  saves  the  use  of  a  straightedge 
if  the  hole  to  be  gaged  is  either  countersunk  or  counterbored. 


GAGES 


189 


Taper  plug  gages  are  usually  provided  with  either  lines  or 
steps  to  gage  not  only  the  diameters  of  the  tapered  hole,  but 
also  their  locations.  A  taper  gage  is  shown  in  Fig.  7.  A  groove 
is  cut  near  the  large  end  of  this  gage  and  the  width  of  this  groove 
indicates  the  limits.  The  gage  must  go  into  the  work  until  one 
edge  of  the  groove  is  flush  with  or  below  the  face  of  the  part, 
while  the  other  edge  must  never  go  in  beyond  the  face  of  the  part. 
Sometimes  steps  are  provided  to  indicate  the  limits  and  at  other 
times,  lines  are  graved  to  serve  the  same  purpose.  If  the  tapered 
hole  is  properly  dimensioned  on  the  component  drawing  so 


T 


+0-000. 

_  o.oio 


|<— 0.745  "MAX. 


_L 

.50o"l;V 

T 


0       0 

f; 

^    ;/ 

1.500 

; 

i 

( 

T 

t 

Machinery 


Fig.  9.     Illustration  showing  how  Compound  Tolerances  are 

involved  when  testing  the  Concentricity  of  a  Hub 

with  a  Hole 

that  no  compound  tolerances  exist,  the  correct  dimensions  of 
the  gage  are  readily  determined.  If  compound  tolerances  exist 
on  the  drawing,  however,  some  arbitrary  method  of  interpreta- 
tion must  be  promulgated  as  otherwise  endless  arguments  will 
ensue  about  the  proper  gage  sizes. 

Application  of  Combination  Plug  and  Snap  Gages.  A  com- 
bination plug  and  snap  gage  is  illustrated  in  Fig.  8.  Such  gages 
may  be  required  for  several  purposes.  They  may  be  used  to  test 
the  concentricity  of  a  hub  with  a  hole,  the  location  of  a  hole 
from  the  edge  of  a  part,  the  depth  of  a  slot  in  relation  to  a  hole 


INTERCHANGEABLE   MANUFACTURING 


etc.  In  determining  the  dimensions  of  such  gages,  compound 
tolerances  are  almost  inevitably  present.  Therefore,  some  ar- 
bitrary method  of  interpreting  the  drawing  must  be  established. 
A  gage  for  testing  the  concentricity  of  the  hub  with  a  hole  will 
be  considered  first. 

Assume  that  the  hub  and  hole  represented  at  A  in  Fig.  9  must 
be  gaged.  The  diameter  of  the  plug  in  this  case  will  be  taken 
as  the  minimum  size  of  the  hole  or  1.500  inches.  If  it  is  con- 
sidered that  the  limits  given  for  the  hole  and  hub  establish  parallel 
zones  of  permissible  variations,  as  shown  at  B,  there  will  be  a 


;»  |< 0.620     WIN. 


0.628  MAX. 


Machinery 


Fig.  10.     Application  of  a  Combination  Plug  and  Snap  Gage  in 
testing  the  Location  of  a  Hole  from  the  Edge  of  a  Part 

minimum  distance  of  0.738  inch  between  the  gaging  parts  of  the 
combination  gage  shown  at  C,  and  a  maximum  distance  of  0.745 
inch.  The  use  of  this  gage  will  then  permit  the  extreme  condition 
of  eccentricity  which  is  shown  at  D.  If  the  diameter  of  the  hole 
is  maximum  and  the  eccentricity  is  at  this  extreme,  the  size  of 
the  hub  will  be  2.987  inches.  If  the  diameter  of  the  hole  is  mini- 
mum, the  size  of  the  hub  will  be  2.983  inches.  The  more  nearly 
concentric  the  hole  and  hub  are  maintained,  the  greater  the 
amount  of  tolerance  which  remains  for  their  diameters.  The 
full  tolerance  on  these  diameters  becomes  available  only  when 


GAGES 


IQI 


they  are  concentric  with  each  other.  It  may  be  pointed  out  that 
the  condition  shown  at  D  does  not  keep  the  parts  within  the 
parallel  zones  shown  at  B.  This  is  true,  and  will  be  found  to 
be  true  wherever  compound  tolerances  are  involved.  It  is  this 
condition  that  makes  it  necessary  to  establish  some  arbitrary 
interpretation  of  the  drawings. 

The  next  example  will  be  of  a  combination  plug  and  snap 
gage  used  to  test  the  location  of  a  hole  from  the  edge  of  a  part. 
The  procedure  to  determine  the  gage  sizes  is  identical  with  the 
foregoing.  A  part  having  a  hole  which  is  to  be  gaged  from  an 


Fig  11.     Simple  Type  of  Contour  Gage  for  checking  the  Uniformity 
of  Contours  or  Profiles 


Fig.  12.     Matching  Gage  used  in  testing  the  Positions  of  Gradua- 
tions on  a  Part 

edge  is  shown  at  A ,  in  Fig.  10.  The  parallel  zones  of  variation 
given  on  the  drawing  are  shown  at  B,  and  the  gage  is  shown  at  C. 
The  diameter  of  the  plug  is  shown  as  the  minimum  size  of  the 
hole.  As  a  matter  of  fact,  the  diameter  of  the  plug  may  be  any 
size  smaller  than  the  hole  in  these  cases,  as  the  gaging  dimen- 
sion is  controlled  by  the  gap  between  the  edge  of  the  plug  and 
the  steps  of  the  arm.  A  plug  of  minimum  size  is  generally  used 
so  that  it  may  also  be  employed  as  the  "go"  gage  for  the  hole. 


IQ2  INTERCHANGEABLE    MANUFACTURING 

A  modification  of  this  gage  is  used  to  test  the  position  of  a  hole 
that  must  be  carefully  located  between  two  edges.  One  side  of 
the  snap  gage  part  is  made  longer  than  the  other  to  detect  the 
side  at  fault  in  case  the  gage  does  not  go  on. 

Contour  or  Profile  Gages.  Contours  or  profiles  are  among  the 
most  difficult  surfaces  to  gage  properly.  A  contour  gage  of  the 
earliest  type  is  shown  in  Fig.  n,  but  this  type  should  be  used  only 
when  accuracy  is  not  important.  The  main  objection  to  this 
form  is  that  it  measures  only  the  shape  of  the  work  and  not  the 


Fig.  13.     Receiving  Gages  having  Holes  or  Openings  Corresponding  to  the 
Shape  of  the  Part  inspected  by  them 

location  of  the  contour.  A  gage  designed  to  overcome  this  ob- 
jection has  guides  for  the  contour  and  locating  points  for  the 
work. 

Contour  gages  of  the  matching  type  are  sometimes  used  when 
great  accuracy  is  not  required.  The  part  is  placed  on  the  gage 
and  its  outline  compared,  either  visually  or  by  a  straightedge, 
with  the  outline  of  the  gage.  In  Fig.  12  is  shown  one  type  of 
matching  gage.  This  is  used  to  test  the  position  of  graduations 
on  a  part.  The  work  is  inserted  in  the  gage  and  the  graduations 
are  compared  visually.  Similar  gages  are  often  used  for  check- 
ing the  shape  of  springs  made  from  flat  stock  and  also  for  check- 
ing the  graduations  on  dials,  etc. 


GAGES 


193 


Receiving  Gages.  The  simplest  form  of  receiving  gage  is  a 
flat  templet  in  which  a  hole  or  opening  is  cut  corresponding  to 
the  form  of  the  part  to  be  inspected.  Such  gages  do  little  more 
than  insure  inter  changeability.  If  the  part  enters,  it  is  not  too 
large,  but  it  is  impossible  to  determine  from  such  a  gage  if  the 
piece  is  too  small.  If  the  piece  does  not  enter,  it  is  too  large. 
It  is  difficult  to  find  the  exact  location  and  amount  of  the  error. 
Gages  of  this  type  are  shown  in  Fig.  13.  In  an  improved  type 


Fig.  14.     Profile  Gage  which  checks  the  Contour  of  the 
Work  by  Flush  Pins 

of  receiving  gage  the  work  is  inserted  in  the  opening  of  the  gage 
and  properly  located  there.  This  opening  is  a  uniform  distance 
from  the  work  so  that  maximum  and  minimum  plug  gages  may 
be  inserted  between  the  work  and  the  gaging  surfaces. 

This  same  principle  may  be  applied  to  the  gaging  of  irregular 
openings  by  making  a  male  profile  a  uniform  amount  under 
size  and  using  limit  plug  gages  to  check  for  errors.  Fig.  14  shows 
another  type  of  profile  gage  which  checks  the  contour  to  definite 
limits.  In  this  case  the  contour  consists  of  several  flat  surfaces 
cut  at  different  angles.  The  part  is  located  by  block  A.  The 
flush  pins  B,  C,  D,  and  E  check  the  various  faces  on  the  piece. 

Dial  Indicator  Contour  Gages.  The  highest  development  in 
gages  for  formed  surfaces  is  doubtless  the  dial  indicator  contour 


194 


INTERCHANGEABLE   MANUFACTURING 


gage,  a  simple  example  of  which  is  shown  in  Fig.  15.  This  type 
of  gage  consists  of  a  baseplate  which  has  mounted  on  it  means 
for  holding  the  piece  to  be  gaged  as. well  as  the  master  form  with 


Fig.  15.     Dial  Indicator  Contour  Gage 


Fig.  16.     Another  Type  of  Dial  Indicator  Contour  Gage 

which  the  piece  being  gaged  is  compared.  The  piece  to  be  gaged 
is  shown  in  position  at  A ,  and  the  master  form  is  directly  beneath 
it.  The  stud  B  is  used  to  locate  the  work.  The  dial  indicator  C 
is  mounted  on  a  baseplate  of  its  own  and  slides  on  the  baseplate 


GAGES 


195 


r^       O 

r^H 


3J    Sj  '3 


5  a 


bo  ^3 


w 

Q  -^ 


3-8 


^H 


196  INTERCHANGEABLE   MANUFACTURING 

of  the  dial  indicator  follows  around  the  master  form,  while  a 
projection  on  the  baseplate  registers  against  the  work.  Such  a 
gage  is  shown  in  Fig.  16.  Otherwise  its  operation  is  the  same 
as  in  the  previous  example.  This  type  of  gage  lends  itself  readily 
to  the  inspection  of  work  having  many  difficult  and  exacting 
requirements.  A  modification  is  shown  in  Fig.  17.  The  piece 
to  be  gaged  is  shown  below  the  gaging  fixture.  This  gage  is 
used  for  inspecting  the  cam  surface,  A .  In  operation,  the  pin  B 
follows  the  cam  surface  A,  while  the  point  C  on  the  dial  indicator 
follows  the  master  cam  D.  Any  variations  are  thus  readily  and 
accurately  detected.  This  type  of  gage  is  not  limited  to  the  gag- 


Fig.  18.    Simple  Form  of  Flush-pin  Gage,  consisting  of  a  Plunger 
which  slides  in  a  Sleeve 

ing  of  contours,  but  may  also  be  used  for  testing  depths,  steps, 
recesses,  etc.,  much  more  readily  than  a  great  number  of  snap, 
plug  and  depth  gages. 

Flush-pin  Gages.  Flush-pin  gages  are  generally  used  for 
tolerances  over  0.002  inch,  especially  in  cases  where  snap  gages 
cannot  be  applied  conveniently.  It  is  possible  to  use  them  for 
smaller  tolerances,  but  it  is  seldom  practicable  in  such  cases  to 
depend  on  the  sense  of  touch.  The  flush-pin  gage  in  its  simplest 
form,  as  shown  in  Fig.  18,  consists  of  a  plunger  which  slides  in 
a  sleeve.  Steps  are  provided,  sometimes  on  the  top  of  the  plunger 


GAGES 


197 


S  "0 

bo   fl 


<D 

^ 


.2H 

4-> 

£    w 


•g  a 


0^    j2 
d,    0) 


bO 


.a    o 

-§'§ 

o> 

O       CH 


o  H 


o    g 

tn     C3 

D     e^ 


198  INTERCHANGEABLE   MANUFACTURING 

be  applied  in  a  great  variety  of  ways.    Fig.  14  shows  its  applica- 
tion to  a  contour  gage. 

The  advantages  of  flush-pin  gages  may  be  briefly  summa- 
rized as  follows:  The  flush-pin  gage  is  the  simplest  form  of  gage 
for  measuring  the  position  of  one  surface  with  reference  to  a 
locating  point,  when  the  relation  is  such  that  a  snap  gage  cannot 
be  used.  Flush-pin  gages  are  subject  to  a  comparatively  small 
amount  of  wear,  and  repairs  are  simple.  Mistakes  in  reading  the 
indications  on  them  are  rare. 


Fig.  21.     Gages  with  Three  Sets  of  Double  Flush  Pins 

Sliding-bar  Gages.  Among  the  gages  made  with  sliding  mem- 
bers, the  sliding  bar  gages  occupy  an  important  place.  In  prin- 
ciple, they  are  similar  to  flush-pin  gages,  but  differ  in  the  method 
of  taking  the  readings  or  indications.  A  common  type  is  similar 
to  a  micrometer  in  general  construction.  On  the  sliding  bar 
is  engraved  a  line,  which  must  be  between  two  steps  on  the  frame 
if  the  part  being  measured  is  acceptable.  Another  similar  ex- 
ample is  shown  in  Fig.  19.  Two  lines  are  engraved  on  the  cylin- 
drical part  of  the  frame,  while  a  single  line  is  engraved  on  the 
slotted  surface  of  the  sliding  arm.  The  arm  swings  out  of  the 
way  to  allow  the  gage  to  enter  the  work.  This  gage  is  used  to 
measure  the  thickness  of  the  bottom  of  a  shell,  and  is  made  light 


GAGES 


IQ9 


for  ease  of  operation,  as  the  work  itself  is  too  heavy  to  be  handled 
rapidly. 

Fig.  21  shows  a  gage  with  three  sets  of  double  flush  pins  for 
measuring  the  irregular  slot  in  the  piece  shown  in  A.  On  sliding- 
bar  gages  where  the  tolerance  is  too  small  to  be  read  from  lines 
engraved  on  the  plunger,  the  movement  of  the  bar  is  multiplied 
by  a  lever  which  points  to  a  graduated  scale  on  the  side  of  the 
gage.  In  this  way  it  is  possible  to  note  quickly  whether  the 
work  is  machined  within  the  requirements  or  not. 

Flat  Depth  and  Length  Gages.  A  simple  type  of  templet  gage 
to  measure  the  depth  of  a  counterbored  hole  is  shown  in  Fig.  22. 


Fig.  22.     Simple  Type  of  Templet  Gage 

One  step  is  used  as  a  "go"  gage,  while  the  other  is  used  as  a 
"not  go"  gage.  A  similar  gage  can  be  used  for  measuring  the 
length  of  a  shoulder.  Fig.  20  illustrates  a  length  gage,  on  which 
the  limits  are  indicated  by  scribed  lines.  This  type  is  really  a 
special  scale  for  measuring  certain  fixed  dimensions. 

Hole  gages.  A  "hole  gage"  is  a  gage  for  testing  the  location 
of  holes  relative  to  each  other  or  to  a  specified  register  point. 
The  gages  measure  the  distances  between  the  outer  and  inner 
points  of  the  peripheries  of  the  holes.  They  are,  in  effect,  func- 
tional gages.  While  the  gage  does  not  actually  measure  the 
center  distances,  it  will  insure  that  the  accepted  parts  may  be  as- 
sembled properly.  When  designing  such  gages,  the  functional 
conditions  of  the  mating  parts  must  be  carefully  checked  and 


200 


INTERCHANGEABLE    MANUFACTURING 


analyzed  to  insure  that  the  results  desired  will  be  secured 
without  imposing  unnecessary  hardships  on  the  manufacturing 
departments.  Attention  is  called  to  the  previous  discussion  of 
these  conditions  in  the  paragraph  " Dimensioning  of  Holes"  in 
Chapter  V. 

This  type  of  gage  comprises  an  almost  infinite  number  of 
designs.  Some  may  be  simple  templets  with  studs  or  bushings 
and  plugs,  while  others  may  be  almost  duplicates  of  the  drill 
jigs  used  in  producing  the  holes,  using  plugs  through  the  bush- 
ings instead  of  drills  and  reamers.  In  fact,  if  the  drill  jig  is  not 


Fig.  23.    Hole  Gages  and  Plugs  used  in  testing  the  Locations  of  Holes  in  an 
Automobile  Transmission  Case 

in  constant  use,  and  is  kept  properly  checked,  the  addition  of 
suitable  plugs  will  make  it  an  extremely  effective  gage.  A  few 
examples  of  gages  of  this  type  will  be  illustrated  to  indicate  their 
wide  variety.  In  Fig.  23  are  shown  two  hole  gages  with  their 
plugs.  These  are  used  for  testing  the  locations  of  the  holes  in 
the  end  of  the  transmission  case  dealt  with  in  Chapter  VIII. 
They  are  examples  of  the  simplest  type  of  hole  gages. 

A  more  elaborate  gage  for  the  same  transmission  case  is  shown 
in  Fig.  24.  The  holes  in  the  shifter  cover  face  of  the  work  are 
tested  with  this  gage  for  their  relation  to  each  other  and  to  the 
main  shaft.  The  transmission  case  is  centered  on  the  arbor  and 
clamped  by  the  hollow  plug  A.  The  stud  B  locates  the  case 
radially.  The  plate  C  is  placed  on  the  shifter  cover  face,  and  is 


GAGES 


201 


located  from  the  central  arbor  by  forks  D  on  the  plate  fitting  in 
the  grooves  E  on  the  arbor.  The  small  plug  F  is  then  used  to 
test  the  locations  of  the  six  small  holes  in  the  shifter  cover  face. 


Fig.  24.     Gage  used  in  testing  Holes  in  a  Transmission  Case,  the 
Work  being  mounted  on  the  Arbor  for  the  Purpose 


Fig.  25.     Another  Type  of  Hole  Gage  which  is  a  Duplicate  of  the 
Drill  Jig  used  in  machining  the  Holes  in  the  Work 

At  the  left  in  Fig.  25  is  shown  a  hole  gage  unloaded,  and  at  the 
right,  the  same  gage  is  shown  with  the  work  in  the  gaging  posi- 
tion. This  gage  is  a  duplicate,  in  .its  general  design,  of  the  jig 
that  is  used  when  drilling  the  holes  in  the  shifter  cover  face. 


202 


INTERCHANGEABLE   MANUFACTURING 


Factors  Involved  in  Gaging  Threads.  At  the  present  time,  a 
wide  difference  of  opinion  exists  as  to  the  proper  method  of  test- 
ing threads.  Owing  to  the  fact  that  a  thread  is  a  complicated 
composite  surface,  and  that  any  composite  surface  is  difficult 
to  measure  readily,  and  also  because  threads  are  employed  in 
so  many  places  for  a  wide  variety  of  purposes,  this  condition  is 
one  to  be  expected.  There  are  three  main  elements  in  a  threaded 
surface:  The  form  of  the  thread,  the  lead  or  pitch  of  the  thread, 


•-M 


Machinery 


Fig.  26.    Diagram  illustrating  the  Meaning  of  the  Terms  used  in 
the  Discussion  on  Thread  Gages 

and  its  diameter.  The  form  of  the  thread  itself  is  composed 
of  several  surfaces.  The  form  of  a  sharp  V-thread  is  composed 
of  two  angular  flanks,  but  this  form  is  theoretical  only.  A  certain 
amount  of  rounding  or  flattening  is  inevitable  at  the  crest  and 
root.  The  form  of  most  other  standard  threads  is  composed  of 
four  surfaces  —  the  crest,  the  root,  and  the  two  flanks.  Thus  a 
thread  is  a  composite  surface,  the  accuracy  of  which  depends 
upon  the  interrelation  of  the  following:  The  diameters  and  form 
of  the  crest  and  root,  the  form  of  the  two  flanks,  and  the  lead. 

There  is  a  broad  general  principle  in  regard  to  limit  gages 
which  should  always  be  kept  in  mind.  Where  compound  tol- 
erances are  not  involved,  a  "go"  gage  with  fixed  measuring 
surfaces  may  check  as  many  dimensions  at  one  time  as  desired, 


GAGES  203 

and  effective  inspection  will  be  secured.  On  the  other  hand, 
an  effective  "not  go"  gage  can  check  only  one  dimension.  By 
effective  inspection  is  meant  assurance  that  specified  require- 
ments in  regard  to  size  are  not  exceeded.  The  above  principle 
must  be  applied  with  common  sense,  as  there  are  a  great  many 
requirements  that  drawings  fail  to  express  clearly.  This  is  es- 
pecially true  in  the  case  of  surfaces  that  are  threaded. 

The  gaging  of  an  externally  threaded  component  will  now 
be  considered.  A  diagram  of  such  a  surface,  illustrating  the 
terms  that  will  be  used  in  the  discussion,  is  shown  in  Fig.  26. 
The  outside,  or  largest  diameter,  will  be  called  the  "major  diam- 
eter." The  smallest  diameter  will  be  called  the  "minor  diam- 
eter." The  top  of  the  thread  will  be  called  the  "crest,"  and 
the  bottom,  the  "root."  The  sides  of  the  thread  will  be  called 
the  "flanks."  The  dimension  taken  square  with  the  axis  of 
the  thread  from  flank  to  flank  at  any  point,  will  be  called  the 
"pitch  diameter."  The  "included  angle"  is  the  angle  between 
the  flanks  of  two  threads,  and  the  "lead"  is  the  distance  from  a 
certain  point  on  one  thread  to  a  similar  point  on  the  next  thread. 

Method  of  Expressing  Tolerances  on  Threaded  Parts.  The 
correct  method  of  gaging  this  thread  consistently  depends  on  the 
manner  in  which  the  tolerances  are  expressed.  In  any  event, 
the  major  and  minor  diameters  should  be  treated  as  independent 
elementary  surfaces.  They  may  be  gaged,  when  necessary, 
either  independently  or  in  conjunction  with  the  other  elements 
on  the  "go"  gage.  The  tolerances  on  the  other  elements  may 
be  expressed  in  one  of  two  ways  —  either  as  a  total  cumulative 
error  expressed  in  terms  of  the  pitch  diameter,  which  will  elimi- 
nate compound  tolerances,  or  as  individual  tolerances  on  each 
element,  which  will  introduce  compound  tolerances  with  all  their 
resulting  annoyances  and  inconsistencies. 

If  the  tolerances  are  expressed  in  the  second  manner,  the 
only  consistent  method  of  gaging  would  be  to  provide  suitable 
gages  for  each  element  and  to  test  each  of  them  independently. 
Any  gage  to  test  all  of  the  elements  at  one  time  would  need  to 
be  a  functional  gage  and  its  design  would  involve  a  careful  study 
of  each  set  of  companion  threads  to  establish  the  proper  dimen- 


204 


INTERCHANGEABLE   MANUFACTURING 


sions.  If  the  tolerances  are  expressed  in  the  first  manner,  the 
gaging  problem  is  simpler.  The  "go"  gage  may  be  a  ring  thread 
gage  checking  the  "go"  dimensions  of  all  elements.  As  a  matter 
of  economy,  in  making  the  gages,  it  is  necessary  to  provide  clear- 
ance at  that  part  of  the  ring  gage  which  would  check  the  major 
diameter,  so  as  to  facilitate  lapping  or  grinding.  This  dimension 
on  the  part  may  readily  be  checked,  when  desired,  by  a  simple 
snap  or  ring  gage. 

A  decided  difference  of  opinion  exists  as  to  the  proper  length 
of  engagement  for  thread  gages.    From  an  academic  viewpoint, 


Fig.  27. 


Standard  Ring  Thread  Gages  and  a  Standard  Plug  Thread 
Gage 


the  gage  should  be  as  long  as  the  effective  length  of  the  thread 
on  the  component.  The  effect  of  the  use  of  longer  or  shorter 
gages  is  to  hold  the  error  in  the  lead  of  the  thread  to  a  lesser  or 
greater  amount.  Many  conditions  exist  where  a  relatively 
large  error  in  lead  is  not  important.  In  such  cases,  a  shorter 
gage  will  make  the  manufacturing  conditions  much  easier,  and 
at  the  same  time  pass  only  satisfactory  parts  to  meet  such  con- 
ditions. For  example,  if  the  length  of  a  gage  is  reduced  to  one- 
half  the  effective  length  of  the  thread  on  the  component,  it  will 
permit  an  error  in  lead  of  double  the  amount  which  would  be 


GAGES  205 

allowed  by  a  gage  which  is  as  long  as  the  full  effective  length  of 
the  thread  on  the  component. 

Theoretically,  individual  "not  go"  gages  should  be  provided 
for  the  major  diameter,  minor  diameter,  and  pitch  diameter. 
Practically,  the  most  severe  requirements  will  usually  be  met 
by  providing  a  "not  go"  gage  for  the  pitch  diameter  only.  This 
would  be  a  ring  thread  gage,  made  to  clear  both  the  major  and 
minor  diameters  of  the  component.  Its  length  must  be  such  that 
it  will  not  engage  over  one  or  two  turns  on  the  component.  As 
a  matter  of  fact,  the  strength  of  the  engagement  of  a  screw  and 
nut  depends  primarily  on  the  amount  of  the  engagement  area 
between  the  threads. 

In  many  cases  suitable  inspection  will  be  secured  by  the  use  of 
a  "go"  ring  thread  gage  which  has  clearance  at  the  major  diam- 
eter of  the  component,  and  a  "not  go"  ring  or  snap  gage  for  the 
major  diameters  of  the  component.  At  A  in  Fig.  27  is  shown  a 
standard  ring  thread  gage.  The  gaging  of  internally  threaded 
components  involves  the  same  problem  as  the  gaging  of  those 
threaded  externally.  A  standard  plug  thread  gage  for  this  pur- 
pose is  shown  at  C.  In  this  case  the  "go"  gages  are  made  to 
clear  the  minor  diameter  of  the  component,  while  the  "not  go" 
gages  are  made  to  clear  both  the  major  and  minor  diameters. 

Types  of  Thread  Gages.  Standard  plug  and  ring  pipe  thread 
gages  are  tapered,  and  on  this  account  one  gage  acts  both  as  a 
"go"  and  as  a  "not  go"  gage.  A  notch  on  the  gage  must  be 
flush  with  the  end  of  the  part  within  a  specified  number  of  turns. 
These  gages  are  made  to  clear  both  the  major  and  minor  di- 
ameters. In  Fig.  28  is  shown  a  "qualifying"  gage  for  the  breech 
mechanism  of  a  large  gun.  This  is  a  case  where  the  thread  must 
start  in  a  certain  specified  position.  A  line  is  graved  on  the 
flange  of  this  gage  which  must  coincide  within  specified  limits, 
when  the  gage  is  screwed  home,  with  another  line  on  the  work. 
The  gage  measures  the  relationship  between  flanks  on  the  work 
and  surface  A  on  the  gage.  This  gage  is  cleared  at  all  points 
except  at  surface  A  and  on  the  thread  flanks,  the  angle  of  the 
threads  being  sufficient  to  center  the  gage  when  it  is  screwed 
home. 


206 


INTERCHANGEABLE   MANUFACTURING 


Wing  and  Indicator  Gages.  Wing  gages  are  used  in  some  in- 
stances where  snap  gages  cannot  be  conveniently  employed. 
These  gages  are  of  the  limit  type  and  have  two  projections  or 
wings.  The  principle  is  that  one  wing  must  pass  the  surface  of 
the  work  to  be  gaged,  while  the  other  must  not.  This  construc- 
tion permits  work  to  be  rapidly  and  accurately  gaged. 

Many  effective  gages  can  be  made  simply  by  providing  suit- 
able stands  or  holding  blocks  for  dial  indicators.  In  one  of  the 
prominent  watch  factories,  a  large  percentage  of  the  gaging 


Fig.  28. 


Qualifying  Gage  for  inspecting  the  Breech  Mechanism 
of  a  Large  Gun 


equipment  is  constructed  in  this  manner.  These  indicators  can 
be  set  up  not  only  to  measure  lengths,  diameters,  etc.,  but  also 
profiles  and  locations.  Several  standard  indicating  gages  are 
now  on  the  market  which  require  very  little  in  the  way  of  hold- 
ing blocks,  etc.,  to  adapt  them  to  measure  a  great  variety  of 
surfaces.  In  Fig.  29  is  shown  an  amplifying  gage,  which  is  very 


GAGES 


207 


rapid  in  operation.  In  Fig.  30  is  shown  an  indicator  used  in  con- 
nection with  bench  centers  for  the  purpose  of  testing  the  con- 
centricity of  a  rifle  barrel  that  is  assembled  with  a  receiver. 


Fig.  29.     Amplifying  Gage  which  permits  the  Rapid  Inspection  of 
a  Large  Variety  of  Surfaces 

Functional  Gages.     A  functional  gage  is  one  that  tests  the 
functional  operation  of  a  component  without  strict  adherence 


208 


INTERCHANGEABLE   MANUFACTURING 


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GAGES  209 

Functional  Gaging  of  Gears.  The  satisfactory  gaging  of  gear 
teeth  is  a  complicated  and  difficult  problem,  if  each  element  is 
tested  independently,  because  of  the  many  factors  involved. 
If  the  testing  is  reduced  to  a  functional  inspection,  however,  the 
problem  becomes  simpler.  The  prime  object  of  gears  is  to  trans- 
mit a  uniform  motion,  and  if  this  result  is  accomplished  it  does 


Fig.  31.    Machine  for  testing  the  Uniformity  of  Motion  between  Two  Gears 

not  matter  what  the  exact  contour  or  dimension  of  any  of  the 
operating  surfaces  may  be.  On  the  other  hand,  if  the  gears  do 
not  accomplish  this  result,  the  only  good  that  knowledge  of  the 
various  discrepancies  does,  is  to  indicate  the  possible  causes  for 
the  error.  This  is  useful  information  in  the  making  of  the  gears, 
but  has  little  value  to  the  inspector  who  must  decide  whether  or 
not  the  gears  are  acceptable. 


2IO 


INTERCHANGEABLE   MANUFACTURING 


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GAGES  211 

method  of  testing  the  backlash  is  to  set  the  gears  in  mesh  at 
their  proper  center  distances  and  actually  measure  the  backlash. 
Often  this  can  be  accurately  measured  with  feeler  gages.  Another 
method  is  to  place  a  piece  of  soft  lead  wire  between  two  teeth, 
run  these  teeth  past  the  center  line,  and  measure  the  thickness  of 
the  deformed  wire. 

Description  of  Gear  Gaging  Machine.    The  principal  operat- 
ing parts  of  a  machine  for  testing  the  uniformity  of  motion  be- 


Fig.  33.     Size  Blocks  being  used  to  check  the  Distance  between 
the  Gaging  Surfaces  of  a  Snap  Gage 

tween  two  gears  are  shown  in  Fig.  31.  This  fixture  not  only 
indicates  the  amount  of  error,  but  also  records  it  on  a  chart  when 
desired.  Its  principle  is  as  follows:  The  gears  are  mounted  on 
arbors  A  and  B,  which  are  spaced  to  the  proper  center  distance. 
Under  each  gear  is  a  plain  disk  the  periphery  of  which  is  ground 
to  the  pitch  diameter  of  the  gear.  On  arbor  A  both  the  gear 
and  disk  C  are  mounted  on  the  same  sleeve;  but  on  arbor  B, 
which  carries  the  indicating  device,  the  gear  is  mounted  on  one 
sleeve  while  disk  D  is  mounted  on  another.  Indicator  E  is 
mounted  on  the  sleeve  which  carries  disk  D,  while  an  arm  which 
engages  with  the  indicator  is  attached  to  the  sleeve  carrying 
the  gear.  Plate  F  which  carries  the  chart  is  mounted  on  the 
fixed  arbor  B. 


212  INTERCHANGEABLE   MANUFACTURING 

It  will  be  seen  that  as  the  two  gears  are  revolved,  the  two 
disks  will  also  revolve,  while  any  differences  in  the  angular  posi- 
tion of  the  gear  on  arbor  B  and  disk  D  will  be  recorded  by  the 
indicator.  If  the  gears  are  correct,  the  disks  of  proper  diameter, 
and  there  is  no  slipping  between  them,  a  perfect  circle  will  be 
developed  on  the  chart.  An  error  in  the  diameters  of  the  disks 
or  slipping  between  them  will  develop  a  spiral  on  the  chart. 
This  makes  it  easy  to  detect  the  errors  due  to  the  testing  machine. 
An  error  of  thirty  seconds  in  the  angular  position  of  the  two 


Fig.  34.     Application  of  Size  Blocks  and  End  Measuring  Attach- 
ments for  checking  the  Diameter  of  a  Plug  Gage 

gears  results  in  a  departure  from  a  smooth  line  of  about  TL  inch 
on  the  chart.  The  indicator  will  read  errors  of  ten  seconds. 

At  A  in  Fig.  32  is  a  chart  obtained  from  two  fifteen- tooth 
gears  cut  with  form-milling  cutters.  These  are  the  two  gears 
shown  in  Fig.  31.  At  B  is  a  chart  resulting  from  two  fifteen- 
tooth  gears  cut  on  a  gear  generating  machine  with  a  straight- 
sided  rack  cutter.  A  chart  from  two  twenty-seven-tooth  gears, 
hardened  and  then  ground  in  a  grinding  machine  of  the  molding- 
generating  type  is  shown  at  C. 

Special  Gages  for  Rapid  Inspection.  Whenever  an  extremely 
large  quantity  of  duplicate  parts  must  be  continuously  inspected, 
special  gaging  devices  and  machines  are  often  designed.  Many 


GAGES  213 

ingenious  indicating  devices  are  used  for  this  purpose.  Often 
a  large  number  of  flush  pins  are  operated  at  one  time,  and  their 
position  is  indicated  by  colored  lights,  electrical  contact  being 
made  according  to  the  location  of  the  end  of  the  flush  pin.  At 


Fig.  35.     Checking  the  Hole  of  a  Ring  Gage  by  Means  of  Size 
Blocks  and  Internal  Measuring  Ends 


Fig.  36.    Employing  Size  Blocks  to  test  the  Distance  between  Two 
Pins  of  a  Gage 

other  times,  several  gaging  devices  are  built  on  a  surface  plate, 
while  the  work  is  moved  readily  from  position  to  position. 
Again,  in  the  case  of  the  inspection  of  cartridges  for  small  arms, 
special  automatic  machines  are  sometimes  built  for  this  purpose 


2I4 


INTERCHANGEABLE    MANUFACTURING 


-machines  which  require  only  the  services  of  an  operator  to 
feed  the  work,  while  the  machine  automatically  gages  and  sorts 


Fig.  37.    Use  of  Size  Blocks  in  Conjunction  with  a  Sine  Square 
for  testing  the  Accuracy  of  an  Angular  Surface 


Fig.  38. 


Illustration  showing  the  Use  of  a  Size  Block  and  Sine 
Surface  Plate  in  testing  a  Tapered  Part 


the  parts.    After  all,  gaging  is  one  of  the  important  operations 
required  in  interchangeable  manufacturing,  and  gaging  equip-. 


GAGES 


215 


ment  should  be  designed  to  meet  the  particular  needs  of  each 
component,  due  attention  being  paid  to  the  same  factors  as  govern 
the  design  of  the  other  manufacturing  equipment. 

Master  Gages  and  Reference  Gages.  It  is  evident  that  some 
means  of  testing  gages  for  their  accuracy  must  be  provided. 
The  extent  of  this  testing  equipment  depends  on  the  ultimate 
accuracy  required.  Gages  for  forgings  or  other  semi-finished 
parts  can  be  effectively  checked  with  standard  measuring  instru- 
ments, such  as  micrometers,  scales,  height  gages,  protractors,  etc. 
On  the  other  hand,  gages  for  intricate  interchangeable  parts 
must  be  checked  with  far  greater  care  and  require  much  better 
measuring  facilities.  Devices  for  checking  elementary  dimen- 


Fig.  39.    Gage  for  testing  a  Composite  Surface,  and   Reference 
Gage  employed  in  its  Checking 

sions,  such  as  length,  diameter,  thickness,  etc.,  will  be  considered 
first.  Measuring  machines  are  on  the  market  for  establishing 
such  dimensions  within  very  small  limits  of  error. 

Accurate  size  blocks  in  sets  which  can  be  built  up  to  any  de- 
sired dimension  are  also  on  the  market.  With  such  equipment, 
it  is  seldom  necessary  to  make  special  master  or  reference  gages 
for  special  dimensions.  The  older  practice  was  to  make  up 
special  size  blocks  for  every  important  gage  —  a  practice  which 
was  very  expensive.  A  set  of  size  blocks,  with  a  few  attachments, 


2l6  INTERCHANGEABLE   MANUFACTURING 

will  cover  a  wide  field  of  checking  and  eliminate  the  need  of  the 
great  majority  of  special  reference  gages.  For  example,  in  Fig.  33, 
several  blocks  are  being  used  to  check  a  snap  gage.  In  Fig.  34, 
with  the  use  of  end  measuring  attachments,  several  others  are 
used  to  check  a  plug  gage.  Again,  in  Fig.  35,  size  blocks  are 
being  used  with  internal  measuring  ends  to  check  a  ring  gage. 
Fig.  36  illustrates  their  use  in  testing  the  location  of  two  pins 
in  a  gage.  A  more  elaborate  set-up  is  shown  in  Fig.  37.  Here 
a  sine  square  and  size  blocks  are  employed  for  testing  the  ac- 
curacy of  an  angular  surface.  Fig.  38  shows  their  use  with  a  sine 
surface  plate  to  test  a  taper. 

In  each  of  the  above  cases,  every  surface  has  been  treated  as 
an  elementary  surface  for  the  purpose  of  testing.  For  the  rapid 
testing  of  composite  surfaces,  however,  special  master  or  ref- 
erence gages  are  required.  Sometimes  these  masters  are  tried 
directly  on  the  gage.  Such  is  the  case  with  the  master  shown 
to  the  right  in  Fig.  39.  In  other  cases,  the  master  is  a  duplicate 
of  the  gage  itself,  and  the  gage  is  tested  by  comparative  meas- 
urements from  the  master.  This  is  common  practice  with  plug 
thread  gages.  A  measurement  is  taken  with  thread  micrometers 
or  over  wires  on  the  master  gage,  and  a  similar  measurement  is 
taken  on  the  gage  itself.  The  comparison  between  the  two 
measurements  determines  the  conditions  of  the  gage. 


CHAPTER  X 
INSPECTION   AND   TESTING 

THE  proper  inspection  of  the  many  parts  of  an  interchange- 
able product  is  one  of  the  most  important  as  well  as  most  dif- 
ficult tasks  in  manufacturing.  On  the  one  hand,  if  inspection  is 
carried  on  in  an  arbitrary  and  exacting  manner,  it  will  retard 
production  and  increase  the  cost  of  manufacture.  On  the  other 
hand,  if  it  is  done  in  a  perfunctory  and  lenient  manner,  it  is 
worse  than  useless,  as  it  will  not  prevent  the  sending  out  of  an 
unsatisfactory  product.  The  principles  of  inspection  formulated 
in  this  chapter,  if  correctly  applied,  will  promote  the  economical 
production  of  parts  on  an  interchangeable  manufacturing  basis 
and  will  insure  that  all  requirements  are  met. 

With  any  system  of  inspection,  the  tendency  of  the  productive 
departments  is  to  neglect  all  thoughts  of  the  ultimate  use  of  the 
parts  in  process  and  to  concentrate  on  efforts  to  increase  pro- 
duction, the  standard  of  quality  apparently  being  anything  that 
the  inspector  will  not  reject.  Thus,  an  unfortunate  situation 
almost  inevitably  arises,  the  production  departments  consider- 
ing only  the  detailed  manufacturing  problems  and  constantly 
seeking  the  line  of  least  resistance  as  regards  the  machining; 
while  the  inspectors  either  connive  at  this  or  arbitrarily  insist 
on  the  exact  letter  of  the  drawings  and  specifications  without 
regard  to  the  ultimate  use  of  the  part  in  question. 

Neither  of  the  above  forms  of  inspection  is  satisfactory.  In 
previous  chapters,  it  has  been  pointed  out  that  seldom,  if  ever, 
are  the  drawings  and  specifications  complete  or  correct  enough 
to  be  followed  blindly.  The  mere  meeting  of  the  drawing  re- 
quirements is  not  in  itself  the  prime  object  of  manufacturing. 
The  main  purpose  in  all  branches  of  interchangeable  manu- 
facturing is  to  promote  the  economical  production  of  satisfactory 
mechanisms.  If  the  inspection  is  to  accomplish  its  part  in  this 

217 


2l8  INTERCHANGEABLE    MANUFACTURING 

work  discretion  should  be  used,  and  due  consideration  should  be 
given  to  many  factors. 

Discrepancy  between  Part  and  Drawing.  When  a  piece  of 
work  is  not  in  accordance  with  the  component  drawing,  any  one 
of  several  conditions  may  be  present:  First,  the  part  may  be 
wrong  and  the  drawing  correct.  Second,  the  part  may  be  correct 
and  the  drawing  in  error.  Third,  both  the  part  and  the  drawing 
may  be  correct.  And  fourth,  both  the  part  and  the  drawing 
may  be  wrong.  In  the  first  case,  if  the  part  cannot  be  salvaged, 
it  must  be  scrapped.  In  the  second  case,  the  part  should  be  ac- 
cepted and  steps  taken  immediately  to  have  the  drawing  cor- 
rected. The  third  case  requires  more  consideration.  For  almost 
every  problem  there  is  more  than  one  satisfactory  solution.  If 
the  part  as  made  can  be  reproduced  at  a  lower  cost  than  one  in 
accordance  with  the  drawing,  the  part  should  be  accepted  and 
the  drawing  corrected  accordingly.  But  if  the  part  as  made  is 
more  difficult  to  reproduce,  the  drawing  should  remain  un- 
changed, although  the  part  need  not  be  rejected.  In  the  fourth 
case,  if  the  part  cannot  be  salvaged,  it  is,  of  course,  scrapped; 
but  the  necessary  corrections  should  be  made  on  the  drawing 
as  soon  as  possible. 

In  all  of  this  work,  due  consideration  must  be  given  to  the 
succeeding  manufacturing  operations  and  the  equipment  pro- 
vided for  performing  them.  When  elaborate  manufacturing 
equipment  is  provided,  it  is  often  cheaper  to  scrap  parts  that 
might  otherwise  be  salvaged,  because  of  the  difficulty  of  com- 
pleting them  with  the  existing  equipment.  It  is  obvious  that 
when  a  great  volume  of  production  is  involved,  discretion  can 
be  exercised  by  but  few  of  the  inspection  personnel.  Hence  any 
abnormal  volume  of  rejections  by  the  inspectors  should  be  the 
occasion  for  a  reinspection  of  the  rejected  parts  by  suitable 
persons  competent  to  locate  the  error,  whether  in  parts  or  draw- 
ings. 

Incomplete  Drawings  and  Specifications.  The  original  draw- 
ings seldom  give  information  complete  in  every  detail  which  is 
necessary  to  produce  every  part.  The  production  departments, 
however,  must  complete  the  parts  with  or  without  the  assistance 


INSPECTION   AND   TESTING  2IQ 

of  the  drawings  and  specifications,  while  the  inspection  depart- 
ment must  decide  whether  or  not  the  parts  thus  produced  are 
satisfactory.  The  first  solution  of  some  of  these  indefinite  points 
may  not  be  the  best  one,  and  several  different  methods  may  be 
tried  out  before  one  that  is  entirely  satisfactory  is  reached.  Dur- 
ing all  this  development,  the  inspector  must  give  a  great  deal 
of  consideration  to  the  problem  so  as  not  to  delay  production 
unnecessarily  and  still  insure  a  satisfactory  product.  As  soon 
as  any  problem  is  solved,  the  inspection  department  should  be 
responsible  for  passing  the  information  along  to  the  proper  per- 
sons so  that  it  may  be  recorded. 

Position  of  Inspection  Department.  This  makes  it  clear  that 
the  inspection  department,  among  its  other  duties,  must  act  as 
eyes  for  the  engineering  department.  For  this  reason,  in  some 
plants  the  inspection  department  reports  to,  and  is  virtually 
a  part  of,  the  engineering  department.  In  other  plants,  it  is  a 
distinct  department  and  is  responsible  to  the  management. 
Either  plan  usually  works  out  well.  In  no  event  should  it  be  a 
part  of,  or  subordinate  to,  any  production  department.  The 
duties  of  the  production  and  inspection  departments  are  so  in- 
compatible that  they  should  never  be  combined,  and  if  they  are, 
one  or  the  other  is  bound  to  suffer.  Nevertheless,  the  inspection 
should  be  carried  on  in  close  cooperation  with  the  production 
departments,  and  all  other  departments  of  the  organization. 

The  mechanical  inspection  of  component  parts  falls  logically 
into  two  main  divisions :  The  first  is  the  shop  inspection  which  is 
performed  while  the  parts  are  in  the  process  of  manufacture. 
The  object  of  this  inspection  is  to  cull  out  defective  work  as  early 
as  possible  and  to  discover  any  defects  in  the  manufacturing 
equipment  which  result  in  faulty  work.  The  second  division  is 
the  final  examination  of  the  completed  parts.  The  object  of  this 
is  to  see  that  all  components  that  will  function  properly  are 
accepted,  and  all  unserviceable  parts  rejected.  There  is  also  the 
inspection,  or  testing,  of  the  assembled  mechanism  to  detect 
faults  that  have  not  been  detected  prior  to  assembling  the  parts. 

Shop  Inspection  Methods.  When  the  production  is  con- 
tinuous, the  shop  inspection  should  be  so  established  that  the 


22O  INTERCHANGEABLE    MANUFACTURING 

parts  are  rigidly  inspected  after  each  •  machining  operation  on 
every  important  functional  surface.  The  requirements  of  these 
surfaces  should  be  definitely  determined  and  recorded  as  early 
as  possible,  and  these  requirements  should  be  rigidly  maintained. 
These  inspections  are  as  important  as  any  of  the  productive 
operations,  and  should  be  maintained  accordingly.  The  in- 
spection of  non-functional  and  other  unimportant  surfaces  can 
usually  be  handled  by  an  inspector  who  passes  from  department 
to  department  and  periodically  checks  a  small  percentage  of  the 
product  at  each  machine.  If  errors  are  discovered,  the  machine 
should  be  stopped  and  the  set-up  corrected,  but  the  parts  at 
fault  should  not  necessarily  be  rejected. 

The  inspectors  are,  of  course,  supplied  with  the  necessary 
gages,  limit  gages  being  essential  for  most  of  the  important  func- 
tional surfaces,  while  "go"  gages  alone  are  usually  sufficient 
for  surfaces  of  lesser  importance.  Definite  lists  of  the  essen- 
tial inspections  or  important  functional  surfaces  should  form 
a  part  of  the  specifications.  These  may  be  combined  with  the 
operation  lists  or  may  be  made  up  as  separate  schedules. 
Whether  or  not  each  individual  piece  of  work  should  be  inspected 
depends  largely  on  the  character  of  the  operation.  On  many 
automatic  operations,  a  percentage  inspection  is  sufficient;  while 
on  most  hand  operations,  where  each  piece  is  handled  individu- 
ally and  the  personal  skill  of  the  operator  is  a  factor,  a  one 
hundred  per  cent  inspection  is  usually  required. 

The  inspection  of  screw  machine  parts  made  from  bar  stock 
is  a  case  where  the  inspection  of  a  minor  percentage  is  sufficient. 
The  practice  in  one  plant  is  as  follows:  Each  automatic  screw 
machine  has,  as  part  of  its  equipment,  several  small  metal  work 
boxes  or  baskets  which  are  numbered  consecutively.  These 
are  used  in  order,  and  about  every  fifteen  minutes  the  one  on  the 
machine  is  removed  and  placed  on  a  bench,  while  the  one  with 
the  next  number  is  placed  on  the  machine.  An  inspector  makes 
periodical  visits  and  inspects  a  few  parts  from  each  basket.  If 
these  are  satisfactory,  all  the  parts  are  removed  and  the  empty 
basket  is  returned  to  the  machine  operator.  If  a  faulty  piece  is 
found  in  any  basket,  the  entire  contents  of  that  basket,  all  sue- 


INSPECTION   AND   TESTING  221 

ceeding  ones,  and  also  the  one  preceding  it,  are  set  aside  for  a 
one  hundred  per  cent  inspection  while  the  machine  is  stopped 
immediately  and  its  set-up  corrected. 

This  general  plan  is  adaptable  to  many  other  operations, 
such  as  sub-press  die  work  and  other  punch  press  operations, 
for  example,  where  the  set-up  and  condition  of  the  tools  alone 
practically  control  the  uniformity  of  the  product.  The  shop 
inspection  should  be  carried  on  as  soon  after  the  machining  op- 
erations as  possible.  In  the  case  of  very  large  parts,  the  piece  is 
often  inspected  before  it  is  taken  from  the  machine,  which  saves 
a  new  set-up  if  corrections  are  necessary. 

Personnel  of  Shop  Inspection.  Much  of  the  detail  work  of 
testing  the  parts  with  gages,  particularly  on  small  work,  requires 
little  mechanical  knowledge  or  skill,  and  is  often  satisfactorily 
performed  by  girls.  When  the  quantity  of  production  is  large, 
and  the  inspections  are  subdivided  into  elementary  tasks,  a 
relatively  small  amount  of  training  will  develop  efficient  detail 
inspectors.  The  supervisor  of  such  work,  however,  should  be 
not  only  a  good  mechanic,  but  also  a  person  well  acquainted  with 
the  requirements  of  the  product.  The  successful  chief  inspector 
must  be  firm  but  diplomatic;  his  duties  must  never  degenerate 
into  faultfinding. 

The  person  in  charge  of  the  shop  inspection  has  one  of  the 
most  difficult  positions  to  fill.  A  great  part  of  his  work  in  pro- 
moting economical  production  is  to  prevent  faulty  parts  from 
being  made.  When  an  operation  is  first  set  up,  the  product 
should  be  checked.  If  it  is  satisfactory,  the  succeeding  parts 
must  be  watched  so  that  the  set-up  may  be  corrected  before  any 
work  is  spoiled.  If  the  first  parts  do  not  meet  the  requirements, 
the  set-up  must  be  corrected  before  production  in  quantity  is 
started.  To  fulfill  these  duties  properly,  without  antagonizing 
those  engaged  in  production,  is  a  delicate  task  and  the  closest 
cooperation  is  necessary.  An  arbitrary  inspector  will  soon  stop 
production  entirely.  On  the  other  hand,  an  inefficient  inspector 
can  soon  ruin  the  reputation  of  the  firm.  A  grave  mistake  on 
the  inspector's  part  in  either  direction  will  inevitably  cause  much 
needless  expense. 


222  INTERCHANGEABLE   MANUFACTURING 

Final  Inspection  of  Work.  In  many  ways  the  function  of  the 
final  inspection  is  quite  different  from  that  of  the  shop  inspec- 
tion. The  latter  deals  with  the  parts  as  they  are  being  shaped 
from  the  raw  material.  Nothing  should  be  left  undone  to  pro- 
mote their  completion  in  accordance  with  the  drawings  and 
specifications.  The  final  inspection,  on  the  other  hand,  deals 
with  the  parts  after  all  the  machining  operations  have  been 
completed.  Its  main  function  is  to  see  that  all  parts  which  will 
give  satisfactory  service  are  accepted,  and  that  those  which  will 
not  are  scrapped.  This  result  alone  should  be  striven  for,  re- 
gardless of  technical  violations  of  the  drawings  and  specifica- 
tions. Under  normal  conditions,  such  violations  should  be  rare. 
If  conditions  are  abnormal,  steps  should  be  taken  to  correct  them, 
but  such  steps  should  not  involve  rejection  of  serviceable  parts. 

In  general,  gages  used  in  the  final  inspection  should  be  func- 
tional gages  only.  If  the  detailed  shop  inspection  is  properly 
organized,  there  is  no  need  to  duplicate  it  here.  If  it  is  not  func- 
tioning properly,  the  trouble  should  be  corrected  at  its  source. 
Both  the  shop  inspection  and  the  final  inspection  should  be  under 
the  general  direction  of  the  same  person.  All  final  decisions  as 
to  the  acceptance  of  questionable  material,  whether  questioned 
by  the  shop  inspection  or  the  final  inspection,  should  also  be 
made  by  the  same  person. 

One  of  the  duties  of  the  inspectors  who  perform  the  final  in- 
spection should  be  to  watch  the  work  of  assembly,  in  addition 
to  testing  the  assembled  product.  If  the  component  parts  as- 
semble properly,  and  the  completed  mechanisms  function  as 
they  should,  no  further  evidence  is  needed  that  the  productive 
work  has  been  properly  done.  If  the  parts  give  trouble  in  as- 
sembling, or  the  mechanism  fails  to  function,  immediate  steps 
should  be  taken  to  locate  the  trouble  and  correct  it  at  its  source. 
Thus  the  assembling  departments  form  the  best  points  of  vantage 
to  watch  and  judge  the  results  that  are  being  obtained. 

Inspection  of  Gages  and  Material.  The  inspection  of  the 
gages  used  in  the  course  of  manufacture  is  one  of  the  vital  func- 
tions of  the  inspection  department.  As  gages  become  worn, 
they  permit  parts  larger  or  smaller,  as  the  case  may  be,  to  pass 


INSPECTION  AND   TESTING  223 

inspection.  The  adjustment  of  a  machine  or  tool  is  not  changed 
as  long  as  the  parts  produced  satisfy  the  gages  used  in  their  in- 
spection, and  so  all  gages  should  be  checked  periodically  and 
corrected  or  replaced  as  found  necessary.  Too  often  this  is  not 
done  until  trouble  has  developed.  One  of  the  principal  objects 
of  inspection  is  not  to  locate  the  cause  of  trouble  after  it  has 
happened,  but  to  forestall  and  prevent  it.  A  systematic  inspec- 
tion of  gages  is  a  sure  means  of  accomplishing  this  end. 

Most  of  the  inspection  of  the  composition  and  physical  strength 
of  materials  belongs  in  the  chemical  and  metallurgical  labora- 
tories. When  a  part  is  subjected  to  unusual  stress,  it  is  customary 
to  make  a  chemical  analysis  of  each  bar  of  stock  as  it  is  received. 
This,  and  other  specified  physical  tests,  are  seldom  considered  in 
connection  with  the  mechanical  inspection  of  the  product.  Often, 
however,  after  such  operations  as  forging,  hardness  tests  are 
made  in  conjunction  with  the  other  inspections  to  insure  that 
the  metal  is  in  proper  condition  to  be  readily  machined.  Such 
tests  are  usually  made  with  simple  testing  instruments,  such  as 
a  Brinell  testing  machine  or  a  scleroscope,  the  mechanical  op- 
eration of  which  requires  no  more  skill  than  the  proper  handling 
of  gages,  and  are,  therefore,  usually  conducted  by  the  regular 
mechanical  inspection  personnel. 

Testing  of  Assembled  Mechanisms.  Whenever  possible,  the 
assembled  mechanism  is  tested  by  actually  having  it  perform  the 
work  it  is  intended  for.  Thus,  a  newly  completed  automobile 
is  usually  sent  out  for  a  road  test  before  being  shipped;  a  type- 
writer is  manipulated  by  an  operator;  a  rifle  is  tried  out  by  a 
marksman,  etc.  These  tests,  of  course,  only  prove  the  condition 
of  the  mechanism  while  new.  The  test  of  service  depends  upon 
the  materials  used  in  its  construction  and  the  honest  workman- 
ship which  has  been  built  into  it  at  every  stage  beginning  with 
the  designing  and  followed  by  the  careful  and  watchful  work  of 
the  productive  operators,  the  vigilance  and  good  judgment  of 
the  inspectors,  and  the  care  and  attention  of  the  assemblers. 
No  one  factor  is  predominant.  All  are  essential  and  each  one 
must  be  carefully  studied  in  order  to  develop  and  maintain  a 
smooth  flow  of  production. 


CHAPTER  XI 
MANUFACTURING   FOR   SELECTIVE   ASSEMBLY 

SELECTIVE  assembly  manufacturing  is  a  method  of  manufac- 
turing which  is  similar  in  many  of  its  details  to  interchangeable 
manufacturing.  In  the  selective  assembly,  the  component  parts 
are  sorted  and  mated  according  to  size,  and  assembled  or  in- 
terchanged with  little  or  no  machining.  Because  of  their  simi- 
larity, the  two  methods  are  often  confused,  and  this  has  led  to 
misapprehensions  in  regard  to  the  principles  of  interchangeable 
manufacturing.  The  production  of  many  commodities  involves 
both  methods  of  manufacture  and  this  has  led  to  even  greater 
confusion.  The  general  principles  of  both  of  these  methods  are 
compared  in  this  chapter  for  the  purpose  of  explaining  the 
principles  involved. 

The  chief  purpose  of  manufacturing,  by  selective  assembly 
or  interchangeable  methods,  is  the  production  of  large  quantities 
of  duplicate  parts  as  economically  as  possible,  within  such  limits 
that  they  may  be  assembled  without  further  machining.  In 
order  to  achieve  this,  close  attention  must  be  paid  to  the  basic 
principles  governing  production,  including  the  design  of  the 
mechanism  and  the  process  of  manufacture. 

The  general  principles  of  design  are  identical  for  manufactur- 
ing on  an  interchangeable  basis  and  on  a  selective  assembly 
basis.  The  functional  design  must  first  be  made  and  tested, 
then  the  manufacturing  design  developed.  This  modifies  the 
inventive  design  so  that  the  product  may  be  manufactured  on  a 
large  scale  in  an  economical  manner.  This  subject  of  design 
was  discussed  in  a  previous  chapter.  One  point,  however,  should 
be  kept  constantly  in  mind.  It  is  seldom  that  every  part  of  a 
mechanism  is  to  be  made  for  selective  assembly.  Usually  a  very 
small  percentage  of  the  parts  are  so  made. 

A  model  mechanism  is  the  representation  of  the  design  in  metal. 
Thus,  as  with  the  design,  the  general  principles  and  purposes  of 

224 


SELECTIVE   ASSEMBLY  22$ 

the  model  are  identical  for  both  interchangeable  and  selective 
assembly  manufacturing. 

Clearances  and  Tolerances  in  Selective  Assembly  Manufactur- 
ing. The  matter  of  clearances  and  tolerances  is  quite  different 
when  manufacturing  on  an  interchangeable  basis  from  when 
manufacturing  on  the  basis  of  selective  assembly.  In  inter- 
changeable manufacturing,  the  minimum  clearances  should  be 
as  small  as  the  assembling  of  the  parts  and  their  proper  opera- 
tion under  service  conditions  will  allow.  The  maximum  clear- 
ances should  be  as  great  as  the  functioning  of  the  mechanism 
permits.  The  difference  between  the  maximum  and  minimum 
clearances  establishes  the  sum  of  the  tolerances  on  the  com- 
panion surfaces. 

When  this  allowable  difference  is  smaller  than  normal  manu- 
facturing conditions  will  permit,  however,  parts  cannot  be  eco- 
nomically manufactured  on  an  interchangeable  basis.  In  such 
cases  one  of  two  courses  is  open.  First,  excess  metal  may  be 
left  on  one  part  which  is  fitted  at  assembly  —  this  usually  proves 
an  expensive  process,  or  second,  tolerances  can  be  established 
which  enable  the  parts  to  be  manufactured  economically  and 
then  sorted  and  assembled  according  to  their  size.  This  second 
method  is  known  as  selective  assembly  manufacturing.  There 
are  several  methods  of  attaining  this  end.  In  general,  the  usual 
method  consists  of  treating  the  more  intricate  companion  parts 
like  interchangeable  parts,  that  is,  making  the  basic  dimen- 
sions on  one  part  represent  the  maximum  metal  conditions,  and 
having  the  tolerances  define  the  minimum  metal  conditions. 
The  extent  of  the  tolerances,  however,  will  be  determined  by  the 
extent  of  the  normal  manufacturing  variations.  The  basic  di- 
mensions of  the  companion  part  would  represent  the  minimum 
metal  condition  —  not  the  maximum  metal  sizes  as  in  inter- 
changeable manufacturing  —  and  the  direction  and  extent  of 
the  tolerance  would  be  identical  with  the  first  piece. 

The  practice,  which  is  correct  for  selective  assembly,  of  making 
the  tolerances  represent  the  normal  variation  of  the  manufactur- 
ing process  employed,  is  often  mistakenly  used  when  manu- 
facturing on  an  interchangeable  basis.  If  such  a  practice  adds 


226 


INTERCHANGEABLE   MANUFACTURING 


)0 

1 

t 

I  II 


J.  MAX.   CLEARANCE  =  0.0002 
2 


Machinery 


Fig.  1. 


Proper  Assembling  Conditions  in 
Selective  Assembly 


nothing  to  the  expense  of  production,  there  is  no  harm  in  em- 
ploying it;  but  too  often  it  imposes  unnecessary  refinement  in 
manufacture,  as  in  almost  every  case,  the  closer  the  tolerances, 
the  more  exacting  and  expensive  will  be  the  manufacturing 
processes.  With  selective  assembly  manufacturing,  on  the  other 
hand,  the  closer  the  tolerances,  the  fewer  the  subdivisions  in  size 
that  will  be  required,  and  the  smaller  the  stock  of  parts  it  is  neces- 
sary to  carry.  This  in- 
troduces a  factor  in  selec- 
tive assembly  manufac- 
turing which  is  not 
present  in  interchange- 
able manufacturing.  The 
economical  balance  be- 
tween the  increased  cost 
of  manufacturing  to  closer 
tolerances  and  the  de- 
creased cost  of  investment 
represented  by  a  smaller  stock  of  different  sized  parts  estab- 
lishes the  proper  course  to  follow  when  manufacturing  on  the 
basis  of  selective  assembly.  Ultimate  economy  here,  as  else- 
where, is  the  main  end  sought. 

Dimensions  and  Tolerances  on  Component  Drawings.  Many 
of  the  general  principles  in  regard  to  component  drawings  for 
parts  made  for  selective  assembly  are  the  same  as  for  inter- 
changeable parts.  Several  details  vary,  however,  due  to  the 
differences  in  treating  the  clearances  and  tolerances.  In  both 
cases  the  effort  should  be  made  to  so  give  the  dimensions  and 
necessary  tolerances  on  the  drawings  that  it  will  be  possible  to 
lay  out  one,  and  only  one,  representation  of  the  maximum  metal 
condition  and  one,  and  only  one,  minimum  metal  condition. 
In  addition  to  this,  for  selective  assembly,  some  notation  must 
be  made  to  indicate  the  proper  grading  and  classification  ac- 
cording to  size.  Thus,  in  selective  assembly  manufacturing,  there 
will  be  a  double  set  of  limits,  the  first  being  the  manufacturing 
limits,  and  the  second  the  assembling  limits. 
Take,  for  example,  the  stud  and  hole  shown  in  Fig.  i,  which 


SELECTIVE   ASSEMBLY 


227 


give  the  proper  assembling  conditions.  The  minimum  clearance 
is  o.oooo  inch  while  the  maximum  clearance  is  0.0004  inch. 
Assume  that  the  normal  manufacturing  variation  on  each  part 
will  be  o.ooio  inch.  Fig.  2  gives  one  method  of  notating  both 
sets  of  limits.  Any  studs  in  Group  A,  for  example,  will  assemble 
in  any  hole  in  Group  A,  but  the  studs  in  one  group  will  not  as- 
semble properly  in  the  holes  in  another  group.  The  example 
stated  shows  one  method  of  grading  parts  when  both  of  the 
companion  parts  are  to  be  sorted  before  assembly.  Many  times 
in  actual  practice,  when  one  of  the  parts  is  complicated,  and  the 
majority  of  its  surfaces  are  interchangeable  ones,  the  minor 


02498"+—:                                              °-2500"-S:o°^ 

WMtm\  . 

X    i                      W///////////S   \4> 

\                    5 

t          'Wffifflfr  t 

GROUP   A 
GROUP   B 
GROUP   C 
GROUP    D 
GROUP    E 

0.2498  IJJSJJS"                       GROUP  A      0.250o"l°;2/'/ 
0.2500  +°-0002r/                        GROUPS      0.2502  +  Q-0QQ2" 
0.2502'±°;^»                      GROUP  c      0.2504  ±;;~g" 

0.2504  i  2:^00  ''            GROUP  D   0-2506"io:oooo'' 

0.2506  ±  J'JJgw                      GROUP  E      0.2508  ioS" 
Moffti/ierv 

Fig.  2.    Information  placed  on  Drawings  used  in  Selective  As- 
sembly Manufacturing  to  facilitate  the  Grading  of  Parts 

part  only  is  sorted  according  to  size.  In  such  cases,  instead  of 
defining  grades  for  the  major  part,  a  note  to  the  following  effect 
is  substituted,  "Select  stud  to  suit  at  assembly." 

In  many  cases,  two  separate  drawings  are  made  of  a  part 
which  is  to  be  graded  before  assembly.  One  shows  the  manu- 
facturing tolerances  only,  so  as  not  to  confuse  the  machine  op- 
erator, while  the  other  gives  the  proper  grading  information. 
In  Chapter  V,  five  laws  of  dimensioning  were  given  for  inter- 
changeable parts.  All  of  them,  except  the  third,  apply  equally 
to  parts  which  are  selectively  assembled.  The  Jaws  which  apply 
to  this  method  of  manufacture  will  be  given  again. 


228  INTERCHANGEABLE   MANUFACTURING 

Laws  of  Dimensioning  for  Selective  Assembly,  i.  In  manu- 
facturing, there  is  only  one  dimension  (or  group  of  dimensions) 
in  the  same  straight  line  which  can  be  controlled  within  fixed 
tolerances.  This  is  the  distance  between  the  cutting  surface  of 
the  tool  and  the  locating  or  registering  surface  of  the  part  being 
machined.  Therefore,  it  is  incorrect  to  locate  any  point  or  sur- 
face with  tolerances  from  more  than  one  point  in  the  same 
straight  line. 

2.  Dimensions  should  be  given  between  those  points  which 
it  is  essential  to  hold  in  a  specific  relation  to  each  other.    The 
majority  of  dimensions,  however,  are  relatively  unimportant  in 
this  respect.    It  is  good  practice  to  establish  common  location 
points  in  each  plane,  and  to  give,  as  far  as  possible,  all  such  di- 
mensions from  these  common  location  points. 

3.  This  law  relates  to  the  proper  basic  dimension  to  be  given 
on  the  component  drawing.    In  selective  assembly  the  conditions 
which  must  be  met  are  so  different,  that  no  general  rule  in  this 
respect  can  safely  be  given.     Each  case  requires  special   con- 
sideration. 

4.  Dimensions  must  not  be  duplicated  between  the  same 
points.     The  duplication  of  dimensions  causes  much  needless 
trouble,  due  to  changes  being  made  in  one  place  and  not  in  the 
others.    It  causes  less  trouble  to  search  a  drawing  to  find  a  di- 
mension than  it  does  to  have  them  duplicated  and  more  readily 
found  but  inconsistent. 

5.  As  far  as  possible,  the  dimensions  on  companion  parts 
should  be  given  from  the  same  relative  locations.    Such  a  pro- 
cedure assists  in  detecting  interferences  and  other  improper 
conditions. 

Similarity  of  Specifications,  Equipment,  Gages,  and  Inspec- 
tion Methods.  The  general  principles  of  specifications  for  inter- 
changeable manufacture,  which  were  given  in  Chapter  VII, 
hold  true  for  manufacturing  on  a  selective  assembly  basis.  Par- 
ticular care  should  be  taken  to  specify  clearly  the  parts  to  be  so 
manufactured  and  the  method  of  grading  to  be  followed.  If  the 
component  drawings  are  properly  made,  there  is  no  difference 
in  the  actual  productive  operations  between  manufacturing  on 


SELECTIVE   ASSEMBLY  2 29 

an  interchangeable  and  on  a  selective  assembly  basis.  In  both 
cases,  the  task  is  to  produce  parts  within  specified  tolerances. 
Therefore,  the  conditions  governing  the  design  of  the  manu- 
facturing equipment  are  constant. 

The  working  and  shop  inspection  gages  for  either  interchange- 
able parts  or  those  made  for  selective  assembly  are  similar.  Ad- 
ditional gages  for  the  purpose  of  grading,  however,  are  required 
for  the  final  inspection.  Often  these  are  indicator  gages,  which 
promote  the  rapid  sorting  of  the  product.  In  other  cases,  gages 
with  successive  steps  or  with  slightly  tapered  measuring  surfaces 
are  used. 

The  detailed  shop  inspection  differs  in  no  particular  from 
that  employed  in  interchangeable  manufacturing.  The  only 
difference  is  the  addition  of  the  selection  and  grading  of  the 
parts  after  completion.  Sometimes  the  actual  selection  takes 
place  at  the  assembly  itself.  If  the  first  part  tried  is  too  large 
or  too  small,  another  is  chosen  which  assembles  properly.  If 
the  rate  of  production  is  relatively  low,  this  procedure  is  often 
satisfactory.  In  fact,  it  is  often  observed  in  the  assembly  of 
parts  which  are  supposed  to  be  interchangeable.  But  if  the  pro- 
duction is  high,  too  much  time  will  be  lost  by  the  assembler  to 
make  the  practice  economical. 

In  general,  manufacturing  on  an  interchangeable  basis  will 
be  found  more  economical  than  manufacturing  on  a  selective 
assembly  basis,  provided  the  design  permits  sufficient  clearances 
to  allow  reasonable  manufacturing  tolerances.  In  the  first  place, 
a  larger  stock  of  parts  is  required  for  selective  assembly  to  in- 
sure that  companion  parts  of  suitable  sizes  will  always  be  avail- 
able. In  the  second  place,  the  additional  expense  of  sorting, 
whether  done  by  an  inspector  or  by  the  assembler,  is  involved 
in  this  method  of  manufacture.  In  its  actual  operation,  the  main 
difference  between  selective  assembly  and  interchangeable  manu- 
facturing is  that  overlapping  tolerances  are  required  in  selective 
assembly  while  such  tolerances  are  absolutely  wrong  in  inter- 
changeable manufacturing. 


CHAPTER  XII 

SMALL-QUANTITY   PRODUCTION    METHODS 

INTERCHANGEABLE  manufacturing  methods,  as  considered  in 
previous  chapters,  relate  to  a  comparatively  high  rate  of  con- 
tinuous production  for  which  the  expense  of  a  complete  equip- 
ment of  special  tools,  fixtures,  and  gages  is  justified,  and  for  which 
the  time  and  constant  study  required  to  keep  the  component 
drawings  in  proper  shape  is  essential  to  prevent  any  break  in  the 
continuous  flow  of  production.  But,  when  any  commodity  is 
manufactured  intermittently  in  small  lots,  the  cost  of  such  pro- 
cedure is  often  greater  than  the  results  justify.  Nevertheless, 
many  of  the  principles  involved  in  interchangeable  manufactur- 
ing can  be  applied  with  economical  results  to  the  production  of 
small  quantities. 

When  comparatively  few  machines  of  one  type  are  manu- 
factured, few  parts  are  duplicated  in  great  numbers,  and  so, 
similar  surfaces,  rather  than  similar  parts,  should  receive  at- 
tention. This  requires  a  thorough  analysis  of  the  four  following 
factors:  (i)  The  possibilities  of  standardizing  the  nominal  sizes 
so  as  to  have  the  smallest  possible  number;  (2)  the  possibilities 
of  standardizing  the  minimum  clearances  between  companion 
parts  for  each  standard  size  to  meet  the  various  functional  con- 
ditions; (3)  the  possibilities  of  standardizing  the  tolerances  for 
the  various  standard  sizes  and  conditions;  and  (4)  the  deter- 
mination of  the  best  surface  to  be  maintained  as  a  standard 
size;  that  is,  whether  it  should  be  the  maximum  male  surface 
or  the  minimum  female  surface.  Until  these  factors  are  deter- 
mined, it  will  be  difficult  to  lay  out  a  simple  and  consistent  pro- 
cedure that  will  result  in  economical  production. 

One  caution  must  be  given  before  further  discussion  is  made 
of  the  subject  of  standardization.  In  order  to  obtain  the  best 
results,  all  known  conditions  involved  must  be  given  due  weight; 
but  the  consideration  given  to  any  factor  should  depend  on  the 

230 


SMALL-QUANTITY   PRODUCTION  231 

frequency  of  its  occurrence.  One  usual  condition  will  far  out- 
weigh several  exceptional  conditions.  An  unusual  condition 
will  always  require  special  consideration  regardless  of  attempts 
at  standardization.  If  an  established  standard  will  not  meet 
the  required  condition,  it  should  not  be  used.  Regardless  of  the 
extent  of  standardization,  exceptions  will  always  occur  and  must 
be  dealt  with.  Thus,  a  standard  is  theoretically  the  best  con- 
struction that  will  satisfy  the  majority  of  the  known  conditions. 
In  practice,  however,  all  existing  conditions  must  be  met.  There- 
fore, if  an  established  standard  is  unsatisfactory  for  any  par- 
ticular service,  unusual  conditions  are  present  and  must  be  met. 

Standardization  of  Nominal  Sizes.  It  is  evident  that  if  the 
number  of  nominal  sizes  employed  is  reduced,  the  number  of 
standard  tools  and  gages  required  in  the  production  department 
will  be  correspondingly  reduced.  As  an  example,  the  matter  of 
reducing  the  number  of  nominal  sizes  of  shafting  was  recently 
taken  up  by  a  committee  of  the  American  Society  of  Mechani- 
cal Engineers,  and  their  recommendations  are  well  worth  follow- 
ing. Two  distinct  but  closely  related  problems  were  considered. 
First,  the  standardization  of  the  diameters  of  shafting  used  for 
the  transmission  of  power,  such  as  lineshafts  and  countershafts, 
etc.  This  usually  consists  of  cold-rolled  shafting  which  is  used 
without  machining.  The  following  fourteen  sizes  have  been 
adopted  as  standard  for  this  type  of  shafting:  Y!~?  IT36"?  IT76?  Itt' 
iyih  2rV  2^,  2f| ,  3^,  3ff ,  4re,  4H»  5r76  and  5^ |  inches. 

The  second  problem  confronted  by  the  committee  was  the 
standardization  of  the  diameters  of  machined  shafting  used  by 
machine-tool  builders  in  making  their  product.  For  this  pur- 
pose, the  following  have  been  adopted  as  standard:  Sizes  up  to 
2\  inches,  increasing  by  intervals  of  sixteenth  inches;  from 
2 \  inches  to  4  inches  inclusive,  by  eighth  inches;  and  from 
4  to  6  inches  by  quarter  inches.  The  foregoing  sizes  are  sufficient 
to  meet  the  majority  of  conditions.  If  proper  attention  is  given 
to  this  point  in  the  design  of  a  mechanism,  the  use  of  unneces- 
sary intermediate  sizes  will  be  eliminated. 

Standardization  of  Minimum  Clearances.  The  amount  of  the 
minimum  clearance  between  companion  parts  depends  on  many 


232  INTERCHANGEABLE    MANUFACTURING 

factors.  Among  them  are  the  size  of  parts,  the  length  of  the 
bearing,  the  class  of  fit  required,  and  the  conditions,  such  as 
temperature,  etc.,  under  which  they  must  operate.  The  classes 
of  fits  which  apply  to  cylindrical  parts,  for  example,  may  be  ap- 
proximately summarized  as  follows:  (i)  Running  fits,  where 
one  part  must  revolve  freely;  (2)  sliding  fits,  where  one  part 
must  slide  freely;  (3)  push,  or  dowel  fits,  where  neither  part  is 
required  to  revolve  but  where  both  parts  must  assemble  readily, 
and  be  held  in  alignment;  (4)  force,  driving,  or  shrinkage  fits, 
which  are  made  with  pressure  or  by  shrinkage,  and  used  in  as- 
sembling parts  which  must  be  held  in  fixed  positions. 

The  amount  of  the  minimum  clearance  for  a  running  fit  is 
dependent,  to  some  degree,  on  the  length  of  the  bearing.  A 
long  bearing,  for  example,  may  have  a  somewhat  greater  clear- 
ance than  a  short  one.  The  proper  length  of  the  bearing  depends 
on  the  load  and  the  material  used  in  the  bearing.  The  load  con- 
trols, to  a  large  extent,  the  diameter  of  the  bearing.  Thus,  the 
first  step  toward  standardizing  the  minimum  clearances  is  to 
determine  the  most  common  material  employed  in  making  the 
bearings,  and  to  establish  standard  lengths  of  bearings  for  the 
various  diameters  of  shafts.  The  exceptions  which  will  inevit- 
ably develop  must,  of  course,  be  treated  on  their  own  merits. 
Take,  for  example,  a  long  feed-screw  or  a  long  bending  roll  which 
is  supported  on  the  ends.  Regardless  of  the  diameter  or  length 
of  the  bearing,  greater  minimum  clearances  than  the  established 
standard  would  be  required.  If  the  number  of  similar  excep- 
tions is  appreciable,  supplementary  standards  can  be  developed 
to  meet  them. 

Another  factor  which  must  be  considered  before  the  stand- 
ards can  be  safely  established,  relates  to  the  conditions  under 
which  the  parts  must  operate.  Thus,  if  the  parts  must  operate 
when  subjected  to  higher  or  lower  temperatures  than  normal 
shop  temperatures,  due  allowance  must  be  made.  On  the  other 
hand,  if  such  temperatures  are  the  exceptions,  the  corresponding 
clearances  must  be  exceptions.  A  good  example  of  this  occurred 
with  a  concern  that  manufactures  power  presses,  several  of  which 
were  ordered  by  a  plant  in  Alaska.  The  shed  in  which  the  presses 


SMALL-QUANTITY   PRODUCTION  233 

were  set  up  was  unheated;  for  this  reason  the  lubricating  oil 
became  very  heavy,  and  the  presses  would  not  work  properly 
until  the  clearances  in  the  bearings  had  been  increased  sufficiently 
to  permit  the  heavier  oil  film. 

In  a  similar  manner,  standards  for  all  other  classes  of  fits 
desired  should  be  developed,  not  only  for  cylindrical  but  also 
for  all  other  common  surfaces.  Every  attempt  should  be  made 
to  standardize  the  more  common  surfaces  and  conditions  first, 
and  the  others  as  it  proves  advisable. 

Standardization  of  Tolerances.  When  manufacturing  inter- 
changeable parts  in  large  quantities,  the  tolerances  should  be  as 
great  as  the  functioning  of  the  mechanism  permits,  in  order  to 
secure  the  greatest  economy  of  manufacture.  As  the  functional 
conditions  will  vary  so  much,  this  practice  seldom  permits  any 
great  standardization  of  tolerances.  On  the  other  hand,  when 
manufacturing  in  small  quantities,  using  standard  tools  and 
equipment  wherever  possible,  the  tolerances  should  represent,  as 
far  as  possible,  results  which  may  be  consistently  obtained  with 
the  use  of  standard  tools  and  which  will  insure  that  the  parts 
will  function  properly.  The  first  step,  therefore,  toward  stand- 
ardizing the  tolerances  is  to  determine  the  accuracy  of  parts  pro- 
duced by  the  various  manufacturing  methods.  In  this  way, 
standard  tolerances  for  each  method  will  be  developed,  such  as 
tolerances  for  grinding,  reaming,  drilling,  boring,  finish-turning, 
rough- turning,  milling,  planing,  etc.  The  next  step  is  to  es- 
tablish a  practice  for  machining  the  various  functional  surfaces 
according  to  the  requirements  which  they  must  meet.  For  ex- 
ample, on  shafts,  all  running  fits  should  be  ground,  all  bearings 
should  be  reamed,  etc.  In  general,  the  extent  of  the  tolerances 
will  increase  as  the  sizes  of  the  parts  increase.  Thus,  for  each 
method  of  manufacturing,  a  standard  tolerance  should  be  de- 
termined for  each  standard  size. 

Maximum  Male  or  Minimum  Female  Size  as  Standard.  In 
considering  whether  the  maximum  male  or  minimum  female 
basic  size  should  be  the  standard,  shafts  and  their  corresponding 
holes  will  be  dealt  with.  In  general,  the  tools  for  making  the 
holes,  such  as  drills,  reamers,  etc.,  are  nonadjustable,  while  the 


234 


INTERCHANGEABLE   MANUFACTURING 


tools  for  machining  the  shafts  are  either  adjustable  in  themselves 
or  are  carried  on  adjustable  members  of  the  manufacturing 
machines.  This  makes  a  strong  argument  in  favor  of  main- 
taining the  basic  size  of  the  holes  as  standard.  This  practice  is 
now  becoming  quite  universal.  Of  course,  an  exception  to  this 
will  always  occur  when  cold-rolled  shafting  is  to  be  used  without 
machining. 

A  little  study  will  show  that  this  practice  can  be  applied  to 
advantage  on  other  surfaces.    The  basic  dimension  of  the  width 


Fig.  1.    Four-speed   10-horsepower   Speed-box   attached   to   the 
Rear  of  a  Radial  Drilling  Machine 

of  a  slot  or  groove  can  be  kept  standard,  and  the  necessary  clear- 
ance can  be  provided  by  reducing  the  size  of  the  mating  member. 

Effecting  Economy  by  Using  Standard  Parts.  The  develop- 
ment and  use  of  standard  parts  offers  one  of  the  greatest  op- 
portunities for  economy  in  the  manufacture  of  small  lots  of  com- 
modities. The  greatest  difficulty  in  the  way  of  accomplishing 
this  is  the  necessity  of  training  the  designers  to  use  -them.  There 
seems  to  be  a  fear  among  these  men  that  the  extensive  use  of  such 
standard  parts  will  limit  their  initiative  and  curtail  their  origi- 
nality. The  fact  is  the  extensive  use  of  standard  parts  will 
eliminate  a  large  amount-of  the  designer's  drudgery,  thus  freeing 
much  of  his  time  and  thought  for  creative  work. 

In  order  to  promote  the  use  of  standard  parts,  records  relating 
to  them  should  be  made  in  a  simple  and  convenient  manner. 


SMALL-QUANTITY   PRODUCTION  235 

Certain  types  of  parts,  such  as  levers,  gears,  bushings,  studs, 
pulleys,  etc.,  should  always  be  considered  as  potential  standard 
parts,  even  if  certain  of  them  are  used  merely  in  a  single  place, 
and  should  be  tabulated  or  indexed  for  ready  reference.  In  this 
way  a  series  of  standard  parts  is  readily  begun.  After  the  start, 
a  study  should  be  made  and  a  balanced  series  laid  out  to  cover 
possible  future  needs.  Otherwise  the  series  will  be  unbalanced, 
that  is,  the  differences  between  some  of  them  will  be  very  small, 


Fig.  2.     Speed-box  applied  to  a  Horizontal  Boring  Machine 

while  between  others  they  will  be  very  great.  Such  a  series  would 
eventually  contain  an  excessive  number  in  order  to  cover  a  given 
range.  The  essence  of  standardization  is  to  reduce  the  number 
of  standards  to  a  minimum. 

Standardizing  Unit  Assemblies  to  Suit  Several  Machines. 
The  design  of  the  commodity  which  is  to  be  manufactured  in 
small  lots  should  be  carefully  studied  and  every  opportunity 
taken  to  incorporate  smaller  unit  assemblies.  As  with  many 
of  the  component  parts,  each  unit  assembly  should  be  considered 
as  a  potential  standard.  If  this  is  done,  many  of  them,  such  as 
oil-pumps,  speed-  and  feed-boxes,  reversing  mechanisms,  etc., 
will  be.  found  applicable  to  several  machines. 


236 


INTERCHANGEABLE   MANUFACTURING 


Fig.  3. 


Application  of  Four- speed  10-horsepower  Speed-box  to 
Another  Horizontal  Boring  Machine 


Fig.  4. 


Large  Blotter  which  is  equipped  with  Same  Speed-box  as 
shown  on  Other  Machines 


SMALL-QUANTITY   PRODUCTION  237 

An  interesting  example  of  what  is  possible  along  these  lines 
is  shown  in  the  accompanying  illustrations.  Here  a  standard 
four- speed,  lo-horsepower  speed-box  is  illustrated  on  various 
types  of  machine  tools.  In  Fig.  i,  this  speed-box  is  shown  at  A 
attached  to  the  rear  of  a  radial  drilling  machine.  In  Figs.  2  and 
-3,  it  is  attached  to  different  types  of  horizontal  boring  machines, 
its  position  again  being  indicated  by  A.  This  speed-box  is  also 
illustrated  in  Fig.  4,  at  A,  as  part  of  a  large  slotter.  The  various 
machines  themselves  are  made  in  small  lots  as  required,  but 
these  speed-boxes,  and  other  common  standard  parts,  are  made 
in  relatively  large  lots  and  carried  in  stock.  This  example  indi- 
cates to  a  small  degree  the  many  possibilities  along  these  lines. 

It  is  of  interest  to  note  in  this  connection  that  one  large  cor- 
poration which  controls  several  plants  building  many  different 
kinds  of  machine  tools,  has  been  carrying  out  a  standardization 
program  for  several  years.  Certain  types  of  parts  and  some  unit 
assemblies  which  have  been  developed  at  the  different  plants 
have  been  compared  and  discussed. 

In  most  cases,  this  discussion  has  led  to  the  adoption  of  a 
certain  series  of  them  as  standards  for  all  plants.  In  addition, 
the  plant  best  fitted  for  that  particular  work  has  been  selected 
to  manufacture  all  of  such  parts  or  unit  assemblies  for  all  the 
plants.  In  this  way,  the  economies  resulting  from  producing 
in  large  quantities  are  secured,  where  no  one  of  the  plants  in- 
volved has  a  very  large  production  of  any  one  size  and  type  of 
machine  tool.  As  with  standard  parts,  if  any  extensive  use  is 
to  be  made  of  standard  unit  assemblies,  they  must  be  recorded 
and  tabulated  in  a  simple  and  convenient  form  for  ready  reference. 

Component  Drawings  for  Small-quantity  Production.  The 
component  drawings  of  a  mechanism  which  is  to  be  manufactured 
in  small  lots  will  vary  considerably  from  those  used  when  the 
production  is  large.  As  a  rule  relatively  few  of  the  operating 
clearances  and  also  few  of  the  manufacturing  tolerances  will  be 
specified.  Notes,  such  as  " force  fit,"  "running  fit,"  etc.,  will 
be  the  usual  method  of  noting  this  information. 

To  determine  properly  the  correct  clearances  and  tolerances 
for  any  surface,  much  time  is  required  for  studying  the  design, 


238  INTERCHANGEABLE    MANUFACTURING 

checking  results  obtained  on  the  various  surfaces  in  production, 
etc.  When  parts  are  made  in  small  quantities,  there  is  little  or 
no  opportunity  to  do  this  work,  although  to  specify  these  re- 
quirements without  such  study  is  generally  useless.  At  best 
they  are  only  a  guess,  and  are  often  established  by  some  one  who 
knows  little  of  the  actual  working  conditions. 

When  a  part  becomes  standard,  or  when  elementary  surfaces 
as  in  holes  and  on  shafts  are  standardized,  the  production  of 
these  parts  and  surfaces  is  large  enough  to  permit  the  necessary 
study  and  tests  to  be  made.  Here  the  component  drawings 
should  specify  the  maximum  and  minimum  sizes  exactly  as  in 
the  case  of  component  drawings  for  large-quantity  production. 
A  full  discussion  of  the  requirements  of  such  drawings  is  given  in 
Chapters  V  and  VI. 

Manufacturing  Equipment.  Relatively  little  special  manu- 
facturing equipment  is  provided  for  manufacturing  in  small  lots. 
Generally  nothing  more  than  boring  jigs  and  planer  templets 
is  necessary.  The  machine  operators  are  usually  skilled  machin- 
ists and  perform  most  of  the  machining  cuts  on  standard  ma- 
chine tools  with  the  use  of  standard  cutting  tools.  Each  piece 
of  work  is  set  up  and  clamped  to  the  bed  of  the  machine  with 
only  the  aid  of  standard  measuring  tools  to  test  its  position. 
With  such  a  type  of  operator,  the  component  drawings  do  not 
actually  require  the  same  amount  of  detailed  information  as  is 
necessary  when  the  work  is  performed  by  less  highly  skilled  labor. 

In  many  cases,  not  even  boring  jigs  or  planer  templets  are 
provided ;  the  work  is  first  laid  out,  and  then  the  machining  cuts 
are  taken  to  match  the  lines  drawn.  As  the  production  increases, 
however,  more  and  more  special  manufacturing  equipment  can 
be  used  to  advantage.  As  the  quantities  become  large  enough 
to  pay  for  the  cost  of  this  equipment,  its  provision  and  use  will 
greatly  promote  economical  production.  The  essential  require- 
ments of  this  equipment,  whether  much  or  little  is  provided,  are 
identical  with  the  requirements  of  equipment  for  manufacturing 
large  quantities. 

Gages  and  Methods  of  Inspection.  The  gages  used  in  this 
type  of  manufacturing  consist  principally  of  standard  measuring 


SMALL-QUANTITY   PRODUCTION  239 

instruments  and  plug,  ring,  and  snap  gages  of  standard  sizes. 
Thread  gages  for  standard  threads  are  also  used  to  an  appreciable 
extent,  as  well  as  adjustable  snap  and  plug  gages  which  may  be 
set  with  the  aid  of  standard  measuring  instruments  or  standard 
size  blocks.  Where  adjustable  gages  are  used,  it  is  very  desir- 
able to  have  means  for  sealing  them  so  that  they  may  be  adjusted 
in  the  tool-room  but  not  promiscuously  in  the  shop. 

When  boring  jigs  are  provided,  these  form  in  themselves  ef- 
fective gages  for  testing  the  location  of  holes  by  using  suitable 
plug  gages  and  bushings  in  place  of  the  boring  tools.  When 
planer  templets  are  employed,  these  also  make  effective  gages, 
an  indicator  being  substituted  for  the  planer  tool.  As  with 
other  manufacturing  equipment,  gages  should  not  be  provided 
until  the  volume  of  production  is  great  enough  to  make  their 
use  economical. 

The  inspection  of  parts  made  in  small  quantities,  where  stock 
is  left  on  many  pieces  for  fitting  at  assembly,  and  where  the 
component  drawings  give  incomplete  information,  is  quite  dif- 
ferent from  the  inspection  of  parts  made  in  large  quantities. 
The  extent  of  the  inspection  required  depends  to  a  large  extent 
on  the  methods  of  paying  the  workmen.  For  example,  when 
the  wage  is  paid  on  a  time  basis  alone,  this  inspection  is  relatively 
slight.  On  the  other  hand,  if  piecework  prices  or  bonuses  are 
paid,  a  more  complete  inspection  is  required,  as  a  bonus  should 
not  be  given  for  spoiled  work. 

Most  of  this  inspection  requires  a  skilled  workman,  as  little 
special  gaging  equipment  is  available,  and  this  necessitates  the 
use  of  standard  measuring  instruments  in  many  cases  and  also 
many  special  set-ups.  In  addition,  with  the  incomplete  com- 
ponent drawings,  the  inspector  must  be  sufficiently  experienced 
to  tell  whether  or  not  the  parts  as  completed  will  function 
properly  when  no  fitting  is  to  be  done  at  assembly.  When  fitting 
is  required,  he  must  also  be  able  to  determine  whether  or  not  the 
amount  of  stock  left  for  this  purpose  is  suitable. 

On  large  pieces,  the  inspection  should  be  made  while  the  part 
is  set  up  on  the  machine  used  in  finishing  it,  so  that  if  corrections 
are  necessary,  they  can  be  made  without  an  additional  set-up. 


240  INTERCHANGEABLE   MANUFACTURING 

When  no  special  locating  fixtures  are  employed  and  a  part  is  re- 
moved from  the  machine,  it  is  almost  impossible  to  relocate  it 
in  order  to  correct  one  surface  and  yet  keep  the  proper  align- 
ment with  the  other  finished  surfaces.  The  inspector  should  be 
capable,  not  only  of  detecting  errors,  but  also  of  convincing  the 
workman  of  them  without  antagonizing  him.  The  inspection 
of  standard  parts  should  be  carried  on  in  the  same. general  man- 
ner as  the  inspection  of  parts  produced  on  a  large-quantity  basis. 
The  assembling  of  small  lots  of  machines  usually  involves  a 
considerable  amount  of  fitting.  For  example,  all  the  small  holes 
are  not  drilled  in  the  larger  pieces  until  assembly.  Small  brackets 
and  similar  parts  are  then  clamped  in  position  and  the  holes  for 
their  holding  screws,  dowels,  etc.,  are  located  from  them.  Slid- 
ing members  are  scraped  to  fit  each  other,  and  to  correct  their 
alignment.  This  requires  a  certain  amount  of  machinery  on  the 
assembling  or  erecting  floor  and  also  the  services  of  skilled  me- 
chanics. However,  as  much  of  the  machining  as  possible  should 
be  completed  before  the  parts  reach  the  assembling  department. 
In  most  cases,  this  requires  the  provision  of  special  manufactur- 
ing equipment  and  gages.  Thus  as  the  quantity  of  the  produc- 
tion increases  and  more  and  more  special  equipment  is  furnished, 
less  fitting  at  assembly  is  necessary.  After  the  machines  are  as- 
sembled, they  should  be  carefully  tested  for  alignment,  back- 
lash, etc.,  and  when  possible,  they  should  be  actually  tried  out 
on  work  of  the  character  they  are  made  to  perform.  This  last  is 
the  crucial  test  because  upon  its  results  the  success  or  failure 
of  the  mechanism  is  judged. 


CHAPTER  XIII 

SERVICE    FACTOR   IN    INTERCHANGEABLE 
MANUFACTURING 

IN  the  final  analysis,  no  manufactured  machine  or  device  is 
ever  purchased  for  itself  alone,  but  is  acquired  for  the  purpose 
of  securing  the  service  which  it  is  supposed  to  render.  Thus, 
for  example,  the  purchase  of  a  reamer  is  the  purchase  of  reamed 
holes  of  a  desired  quality  or  standard.  Consequently  it  follows 
that  the  reamer  which  produces  the  most  reamed  holes  of  the  re- 
quired accuracy  at  the  least  ultimate  cost  is  the  best  reamer 
and  is  finally  recognized  as  such.  Its  first  cost  may  be  higher 
than  others,  yet  if  it  produces  more  holes  during  its  life,  or  pro- 
duces them  more  quickly  or  with  less  power,  the  average  cost  of 
the  reamed  holes  may  be  much  less  than  those  produced  with  a 
cheaper  tool.  In  like  manner,  the  purchase  of  a  machine  tool 
represents  the  purchase  of  machined  surfaces;  the  purchase  of 
a  typewriter,  typed  letters;  the  purchase  of  a  sewing  machine, 
sewn  seams;  of  an  automobile,  transportation,  etc. 

The  ultimate  test  of  any  manufactured  article  is  the  test  of 
service.  The  component  parts  may  be  absolutely  interchange- 
able, the  manufacturing  processes  may  be  developed  to  produce 
large  quantities  economically,  and  the  inspection  may  be  as 
rigid  as  possible;  yet  if  the  required  service  is  not  rendered,  all 
of  this  work  is  useless. 

Preparing  Functional  and  Manufacturing  Designs.  If  a  com- 
modity is  to  give  satisfaction,  this  service  must  be  built  into  it 
at  every  stage  of  its  development  and  manufacture.  The  first 
conception  of  a  new  mechanism  develops  from  the  realization 
of  some  service  to  be  performed.  The  functional  design  of  this 
mechanism  is  solely  the  development  of  some  mechanical  means 
of  performing  this  service;  this  thought  is  paramount  and  every 
other  consideration  is  subordinate  to  it.  It  is  only  after  this  re- 

241 


242  INTERCHANGEABLE   MANUFACTURING 

suit  has  been  obtained  that  any  great  thought  is  given  to  the 
matter  of  producing  the  mechanism  commercially. 

The  primary  purpose  of  the  manufacturing  design  is  to  de- 
velop the  functional  design  into  one  which  can  be  economically 
manufactured;  yet,  at  the  same  time,  the  greatest  care  must 
be  exercised  to  maintain  all  the  serviceable  qualities  of  the  origi- 
nal design.  The  factor  of  economical  manufacture  must  never 
be  the  controlling  one  when  economy  is  secured  at  the  expense  of 
service  rendered.  The  customer  is  purchasing  this  service,  and 
any  action  which  may  rob  him  of  some  part  of  it  is  unjustifiable. 
The  development  of  the  correct  manufacturing  design  is  a  long 
process.  There  are  no  laboratory  tests  which  will  show  all  the 
requirements  and  results  of  service.  The  largest  part  of  this 
information  must  necessarily  come  from  the  study  of  the  results 
obtained  from  the  commodity  when  in  actual  service. 

A  large  amount  of  this  information  can  be  readily  secured 
if  proper  attention  is  given  to  every  complaint  from  customers. 
Too  often,  information  from  such  sources  is  treated  as  an  an- 
noyance to  be  smoothed  over  rather  than  as  a  definite  problem 
to  be  solved  or  as  a  matter  which  is  of  far  more  value  and  im- 
portance to  the  producer  than  to  the  customer.  In  general, 
a  complaint  from  a  customer  results  from  one  of  three  causes: 
First,  some  faults  in  design,  workmanship,  or  material  may 
exist  in  the  mechanism  which  prevent  it  from  giving  the  service 
which  is  due  the  customer.  If  this  is  the  case,  prompt  steps 
should  be  taken  to  correct  the  trouble  at  its  source.  Obviously 
this  matter  is  of  more  importance  to  the  producer  than  to  the 
user  if  he  hopes  to  remain  long  in  business.  Second,  the  cus- 
tomer may  not  thoroughly  understand  the  handling  and  care 
which  the  mechanism  requires.  In  such  cases  it  is  of  the  great- 
est importance  to  the  manufacturer  that  the  customer  obtains 
the  needed  information,  or  else  the  reputation  of  his  product 
will  inevitably  suffer. 

The  third  complaint  is  usually  due  to  the  customer's  attempt 
to  perform  work  for  which  the  product  was  not  intended  or 
which  is  beyond  its  capacity.  It  is  essential  that  the  manu- 
facturer know  the  limitations  of  his  product.  Furthermore, 


SERVICE   FACTOR  243 

information  derived  from  complaints  of  this  sort  often  leads  to 
modifications  of  the  product  which  greatly  increase  its  field  of 
usefulness.  Complaints  of  all  sorts  should  be  carefully  checked 
and  acted  on  accordingly.  Several  manufacturing  concerns  have 
a  man  or  division  in  their  engineering  department  that  in- 
vestigates all  complaints  from  customers,  using  the  information 
so  gained  in  the  improvement  of  their  product.  Only  by  such 
knowledge  of  actual  results  obtained  under  many  conditions  can 
the  maximum  service  be  built  into  a  product. 

Keeping  Specifications  up  to  Date.  The  specifications  should 
include  all  information  which  is  needed  to  produce  a  commodity 
capable  of  giving  the  desired  service.  In  whatever  form  they  are 
kept,  they  should  be  constantly  revised  to  keep  abreast  to  the 
needs  of  service.  For  example,  if  the  material  specified  for  a 
certain  part  proves  too  weak  in  actual  use,  it  must  be  altered. 
Thus,  the  part  may  require  a  stronger  material,  a  different  kind 
of  heat-treatment,  or  a  strengthened  design.  Often  it  may  be 
found  that  the  original  requirements  of  many  surfaces  or  parts 
are  not  the  correct  ones.  All  of  this  information,  if  kept  in  such 
form  that  it  is  always  available,  will  be  found  invaluable  in  the 
development  of  future  products;  products  which  will  contain 
from  the  start  a  higher  quality  of  service  than  any  of  the  pre- 
ceding ones. 

Planning  Production  to  Obtain  Requisite  Service.  Every 
part  of  the  manufacturing  equipment  provided  should  be  selected 
or  designed  with  the  object  of  producing  parts  capable  of  render- 
ing the  required  service.  The  design  of  the  commodity  itself, 
if  properly  recorded  on  the  drawings,  will  emphasize  these  points; 
yet  a  careful  check  should  be  made  to  insure  that  no  vital  factor 
has  been  overlooked. 

The  constant  care  which  must  be  exercised  in  every  stage  of 
the  actual  production  determines  in  a  large  measure  the  character 
of  the  service  delivered.  No  operation  is  too  unimportant  to  be 
neglected.  This  care,  however,  must  be  taken  by  each  individual 
workman.  To  obtain  the  necessary  cooperation,  every  effort 
must  constantly  be  made  to  develop  in  each  workman  the  spirit 
of  true  craftsmanship.  A  craftsman,  in  the  opinion  of  the  writer, 


244  INTERCHANGEABLE   MANUFACTURING 

is  a  man  who  takes  pride  in  the  work  and  skill  of  his  hands  and 
brain;  who  feels  that  each  result  of  his  labor  is  a  monument  to 
himself;  and  whose  enthusiasm  and  consciousness  of  power  pre- 
vent him  from  doing  any  work  but  his  very  best.  No  man  can  do 
justice  to  his  own  capabilities  unless  he  is  interested  in  and  proud 
of  the  results  of  his  labor.  The  manufacturer  must  realize  that 
he  should  have  a  vital  interest  in  the  proper  training  of  each 
one  of  his  workmen,  and  should  use  every  means  in  his  power 
to  foster  true  craftsmanship  in  all  branches  of  his  establishment. 
No  part  of  any  work  is  too  elementary  to  justify  such  an  attitude. 

Inspecting  Parts  to  Insure  Service.  Every  inspector  must 
keep  in  mind  at  all  times  the  requirements  of  service  which  the 
parts  under  inspection  must  render.  This  service  is  the  sole  pur- 
pose for  which  the  parts  are  made.  If  they  will  render  it,  the 
parts  are  correct;  if  not,  they  are  incorrect.  In  a  well-balanced 
organization,  the  inspection  is  not  carried  on  to  discover  the 
faults  which  others  have  committed,  but  rather  to  protect  the 
customer  and  the  firm's  good  name  as  well,  by  guarding  against 
the  possibility  of  faulty  work  going  out  despite  all  precautions 
taken  in  the  productive  departments.  Yet,  even  with  the  most 
rigid  inspection,  some  flaws  remain  hidden  and  are  not  discovered 
until  the  commodity  is  in  the  hands  of  the  customer.  With  an 
honest  inspection,  such  occurrences  will  be  the  exception,  but 
without  proper  safeguards,  these  occurrences  are  apt  to  be  the 
rule,  and  the  customer  will  soon  learn  it,  to  the  disadvantage  of 
the  manufacturer. 

The  majority  of  mechanical  products  are  tested  on  work  of 
the  type  they  are  built  to  perform,  before  they  are  shipped. 
Needless  to  say,  no  attempt  should  be  made  to  favor  the  com- 
modity in  such  tests.  Every  effort  should  be  made  to  detect 
any  faults,  and  each  fault  detected  should  be  permanently  cor- 
rected. The  interest  of  the  manufacturer  in  the  commodity 
should  not  cease  when  it  reaches  the  customer.  It  is  of  more 
interest  to  the  manufacturer  than  to  the  purchaser  to  see  that 
his  product  is  employed  on  the  work  it  can  best  perform,  and  to 
see  that  it  performs  its  maximum  service.  By  so  following  up 
his  product,  he  not  only  makes  a  satisfied  customer  but  also 


SERVICE   FACTOR  245 

creates  new  markets  for  his  product.  Furthermore,  as  noted 
previously,  the  information  gained  by  observing  his  mechan- 
isms in  service  under  many  varying  conditions  will  be  invaluable 
to  him  in  developing  and  improving  his  product,  as  well  as  often 
pointing  the  way  to  the  development  of  new  products. 

Manufacturers  in  a  number  of  different  lines  have  established 
well-organized  service  departments,  with  a  view  to  insuring  that 
the  machines  or  devices  that  they  build  will  give  the  highest 
possible  service  and  satisfaction  to  their  customers.  Such  serv- 
ice departments  are  well  known  in  the  automobile  and  type- 
writer fields,  but  similar  departments,  somewhat  different  in 
their  nature,  on  account  of  the  varying  conditions  under  which 
the  product  is  used,  are  also  found  in  the  machine  tool  field, 
where  some  manufacturers  have  highly  organized  service  de- 
partments for  determining  the  best  conditions  under  which 
the  customer's  work  may  be  performed.  Through  such  service 
departments  it  has  often  been  found  possible  to  increase  greatly 
the  output  of  the  machines  built. 


INDEX 


PAGE 

Accuracy,  definition 27 

required,  of  gages 1 79 

Alignment  and  concentricity 74 

Assembled  mechanisms,  testing 223 

Assembly,  selective,  manufacturing  for 224 

Atmospheric  fits,  definition 24 


Basic  dimensions,  exception  to  general  rule  for 81 

Basic  size,  definition 20 

Chip  clearances,  proper,  necessity  for 134 

results  obtained  with 136 

Clamping  devices,  pneumatic 171 

Clearances 4 

maximum,  definition 22 

minimum,  definition 21 

minimum,  in  small-quantity  production,  standardization  of 231 

proper  chip,  necessity  for 134 

proper  chip,  results  obtained  with 136 

Clearances  and  tolerances 35 

in  selective  assembly  manufacturing 225 

Clearance  surfaces,  definition 24 

Component  drawings 6 

definition 27 

dimensions  and  tolerances  on 226 

examples  illustrating  practice  in  making 77 

for  small-quantity  production 237 

principles  in  making 46 

Compound  tolerances 58 

definition 26 

dimensioning  to  prevent 93 

Composite  surfaces,  definition 25 

dimensioning 56 

Concentricity  and  alignment 74 

Contour  or  profile  gages 192 

Costs,  distribution  of,  in  economical  production 109 

clerical  and  accounting  work 118 

educational  department 112 

employment  department 112 

247 


248  INDEX 

PAGE 

Costs,  factory 109 

general  product  charges 1 16 

health  and  safety  of  employes,  maintaining 113 

inspection  and  testing  of  product 1 20 

interest  on  investment,  depreciation  and  insurance 114 

lack  of  work  or  of  labor 115 

payroll,  making  up 112 

power  charges 114 

records,  keeping  of 115 

specific  product  charges 1 16 

supervision 112 

Cutting  tools  and  sharp  corners,  protection  from 132 

Cutting  tools,  tolerances  allowed  on 141 

Data,  manufacturing,  general 109 

specific 108 

Definitions  of  terms  in  interchangeable  manufacturing 18 

accuracy 27 

atmospheric  fits 24 

basic  size 20 

clearance,  maximum 22 

clearance,  minimum 21 

clearance  surfaces 24 

component  drawings 27 

composite  surfaces 25 

compound  tolerances 26 

elementary  surfaces 25 

function ,. 19 

functional  surfaces 23 

interference 22 

limit 19 

maximum  metal  size ' .  . .  21 

minimum  metal  size 21 

model  size 20 

operating  surfaces 23 

operation  drawings 28 

precision , 26 

register  or  working  points 26 

selective  assembly 18 

tolerances 19 

tolerances,  compound 26 

unit  assembly 26 

Design,  classes  of .- 31 

effect  of,  on  successful  interchangeability 3 

functional  and  manufacturing,  preparing 241 

function  of 30 

manufacturing,  functioning  tested  by  manufacturing  model 40,  44 


INDEX  249 

PAGE 

Design,  of  fixtures,  efficient,  examples  of 130 

of  jigs  and  fixtures 125 

simplifying 32 

Designing  fixtures,  important  factors  in 1 2O 

Designing  for  assembly  and  service 38 

Dial  indicator  contour  gages I93 

Dimensioning,  careless,  possibility  of  draftsman's  errors  increased  by 53 

component  drawings,  examples  illustrating  practice 77 

composite  surfaces 56 

force  fits 61 

for  selective  assembly,  laws  of 228 

holes 65 

in  interchangeable  manufacturing,  laws  of 48 

laws  of,  violations 49 

profile  surfaces 64 

to  prevent  compound  tolerances 93 

Dimensions,  basic,  exception  to  general  rule  for 81 

on  component  drawings 8 

Dimensions  and  tolerances  on  component  drawings 226 

Drawings,  component 6 

discrepancy  between  part  and 218 

functional 47 

manufacturing 47 

Drawings  and  specifications,  incomplete 217 

Drilling  and  milling  fixtures 175 

Drilling,  reaming  and  milling  operations,  examples  of,  in  interchangeable 

manufacture 157 

Economy,  in  interchangeable  manufacturing i,  105 

in  production 105 

Educational  department,  cost  of " 112 

Elementary  surfaces,  definition 25 

Employment  department,  cost  of 112 

Equipment  for  interchangeable  manufacture 14,  121 

Examples,  illustrating  practice  in  making  component  drawings 77 

of  efficient  fixture  design 130 

of  special  equipment  for  interchangeable  manufacture 145 

Expenses,  due  to  direct  labor 112 

indirect,  belonging  to  general  productive  equipment 114 

indirect,  due  to  product "6 

indirect  factory,  distribution  of 1 1 1 

Experimental  model,  function  of 3 


Facing  bar,  example  of  use  of,  in  interchangeable  manufacture 173 

Factory  cost  of  production 109 

Factory  expenses,  indirect,  distribution  of in 


250  INDEX 

PAGE 

Fixtures  and  jigs,  accessibility  of  locating  points 134 

checking  and  testing I4I 

simplicity  and  standardization  of 138 

Fixtures,  designing,  important  factors  in 1 29 

design  of,  in  interchangeable  manufacturing 14 

design  of  jigs  and I25 

efficient  design  of,  examples 130 

methods  of  manufacturing I38 

milling  and  drilling 175 

tolerances  indicated  on  drawings 140 

Flat  depth  and  length  gages 199 

Flush-pin  gages 196 

Force  fits,  dimensioning 61 

Functional  designs,  preparing 241 

Functional  drawings,  purpose  of 47 

Functional  gages : 207 

Functional  surfaces,  definition 23 

Function,  definition 19 

Function  and  essential  requirements  of  product,  indicated  in  specifications.  106 

Gage  requirements  controlled  by  ultimate  economy 182 

Gages,  classified  according  to  use „ 1 79 

combination  snap  and  plug,  application  of 189 

dial  indicator  contour 193 

flat  depth  and  length 199 

flush-pin 196 

for  checking  parts  in  interchangeable  manufacture 12 

functional 207 

hole 199 

in  interchangeable  manufacturing 177 

inspection,  requirements  of " 54 

location  of  holes  checked  by 67 

master  and  reference 215 

plug 186 

profile  or  contour 192 

receiving 193 

required  accuracy  of 179 

ring 185 

sliding-bar 198 

snap 183 

special,  for  rapid  inspection 212 

thread,  types  of 205 

wing  and  indicator. 206 

Gages  and  material,  inspection  of 223 

Gages  and  methods  of  inspection  in  small-quantity  production 239 

Gaging  of  gears,  functional 209 

machine  for..  211 


INDEX  251 

PAGE 

Gaging  threads,  factors  involved  in 202 

Gears,  functional  gaging  of 209 

machine  for  gaging 211 

tolerances,  specifying 76 

Hole  gages 199 

Holes,  dimensioning 65 

expressed  tolerances  on  location  of 70 

location  of,  gages  for  checking 67 

tolerances  for  "group"  location  of 72 

Indicator  gages 206 

Inspection,  of  gages  and  material 222 

of  parts  to  insure  service 244 

of  product,  final 16 

of  product,  in  the  shop 16 

of  work,  final 222 

rapid,  special  gages  for 212 

shop,  personnel  of 221 

Inspection  and  testing 217 

of  product 1 20 

Inspection  and  working  gages,  relation  between 181 

Inspection  department,  position  of 219 

Inspection  gage  requirements 54 

Inspection  methods  and  gages  in  small-quantity  production 238 

Inspection  methods,  shop 219 

Interchangeable    manufacturing,  compared  with  selective  assembly  manu- 
facturing    228 

economical  production  in 105 

equipment  for 121 

examples  of  special  equipment  for 145 

machine  design 29 

service  factor  in 241 

specifications  for 10 

Interchangeable  principle,  applying 35 

Interchangeability,  between  parts  made  in  different  shops 182 

definition 18 

desirable  extent  of 2 

effect  of  design  on 3 

Interference,  definition 22 

Jigs,  examples  of  use  of,  in  interchangeable  manufacture ....    147,  162,  169,  175 

Jigs  and  fixtures,  accessibility  of  locating  points 134 

checking  and  testing 141 

design  of 1 25 

simplicity  and  standardization  of i38 


252  INDEX 

PAGE 

Labor,  direct,  expenses  due  to 112 

lack  of,  effect  on  cost  of  production 115 

skilled  and  unskilled,  value  of 15 

Laws  of  dimensioning,  in  interchangeable  manufacturing 48 

violations  of 49 

Length  gage 199 

Limit  and  tolerance,  definitions 19 

Machine  design  in  interchangeable  manufacture 29 

Machine-hour  rate 114 

Machine  tools,  selection  of 121 

Manufacturing,  causes  of  variations  in 66 

data,  general 109 

data,  specific 108 

design,  preparing 241 

drawings,  purpose  of 47 

equipment  for  small-quantity  production 238 

model,  functions  of 4 

model,  to  test  functioning  of  manufacturing  design 40 

tolerances 5 

Manufacturing  fixtures,  methods  of 138 

Master  gages  and  reference  gages 215 

Materials,  factors  governing  choice  of 32 

cost 33 

machining  qualities 33 

service  required 34 

source  of  supply 33 

weight  of  finished  product 34 

Maximum  metal  size,  definition 21 

Milling  and  drilling  fixtures 175 

Milling  and  profiling  machines,  example  of  use  of,  in  interchangeable  manu- 
facture   : 154 

Milling  operations,  example  of,  in  interchangeable  manufacture 158 

Minimum  metal  size,  definition 21 

Models,  for  standard  of  precision 43 

in  interchangeable  manufacturing,  purpose  of 40 

tolerances  tested  by 42 

Model  size,  definition 20 

Operating  surfaces,  definition 23 

Operation  drawings,  definition 28 

Payroll,  cost  of  making  up 112 

Plug  and  snap  gages,  combination,  application  of 189 

Plug  gages 186 

Pneumatic  clamping  devices 171 


INDEX  253 

PAGE 

Precision,  definition 26 

model  for  standard  of 43 

Production  problems 15 

Product  overhead 116 

Profile  surfaces,  dimensioning 64 

Reaming,  drilling  and  milling  operations,  example  of,   in   interchangeable 

manufacture 157 

Receiving  gages 193 

Reference  gages r 215 

Ring  gages 185 

Safety  and  health  of  employes,  cost  of  maintaining 113 

Selective  assembly,  definition 18 

manufacturing  for 224 

Selective  assembly  manufacturing  compared  with  interchangeable  manu- 
facturing    228 

Service  factor  in  interchangeable  manufacturing 241 

Service,  inspection  of  parts  to  insure 244 

requisite,  planning  production  to  obtain 243 

Shop  inspection,  methods 219 

personnel  of 221 

Sliding-bar  gages 198 

Small-quantity  production  methods 230 

Snap  and  plug  gages,  combination,  application  of 189 

Snap  gages 183 

Specifications,  for  interchangeable  manufacture 10 

function  and  requirements  of  product  indicated  in 106 

keeping  up  to  date 243 

specific  and  general  information 1 20 

Specifications  and  drawings,  incomplete 218 

Standardization,  of  minimum  clearances  in  small-quantity  production 231 

of  nominal  sizes,  in  small-quantity  production 231 

of  parts 37 

of  tolerances  in  small-quantity  production 233 

Standardization  and  simplicity  of  jigs  and  fixtures 138 

Standardizing  unit  assemblies  to  suit  several  machines 235 

Standard  parts,  economy  effected  by  use  of,  in  small-quantity  production ....  234 

Standard  sizes,  based  on  maximum  plug  or  minimum  hole  diameters 233 

Supervision,  cost  of 112 

Terms  used  in  interchangeable  manufacture,  definitions  .  . . 18 

Testing  and  checking  jigs  and  fixtures 141 

Testing  and  inspection 217 

Testing  assembled  mechanisms 223 

Threaded  parts,  tolerances  on,  method  of  expressing 203 

Thread  gages,  types  of 205 


254  INDEX 

PAGE 

Threads,  gaging,  factors  involved  in 202 

Tolerance,  allowed  on  cutting  tools 141 

compound 58 

compound,  definition 26 

for  "group "  location  of  holes 72 

maintaining,  on  tools  machining  several  surfaces 143 

manufacturing 5 

manufacturing,  reduced  by  careless  dimensioning 53 

on  fixture  drawings 140 

on  threaded  parts,  method  of  expressing 203 

specifying,  for  gears 76 

standardization  of,  in  small-quantity  production 233 

tested  by  models 42 

Tolerance  and  limit,  definition 19 

Tolerances  and  clearances 35 

in  selective  assembly  manufacturing 225 

Tolerances  and  dimensions  on  component  drawings 226 

Tools,  cutting,  and  sharp  corners,  protection  from 132 

cutting,  tolerances  allowed  on 141 

for  machining  several  surfaces,  maintaining  tolerances  on 143 

machine,  selection  of 121 

wear  on  cutting  edges  of,  results  of 128 

Unit  assembly,  construction,  advantages  of 37 

definition 26 

standardizing,  to  suit  several  machines 235 

Wing  and  indicator  gages 206 

Working  and  inspection  gages,  relation  between 181 

Work,  inspection  of,  final 222 

Working  or  register  points,  definition 26 


Ft  '^v 


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Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 


12M50RM 


LD  21-1007n-ll,'49(B7146sl6)476 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


